Ok. So as your build already has a lot of set feats and such that you have to take we want to build upon that instead of trying anything new. So let’s take a look at feats and magic weapon enchants that effect critical hits or ways to free up any feats.
Not all of these options are possible for your exact build but I will add them for anyone in the future who wants to improve the build and has different stats than yourself.
Feats
It might be a little late as you have already taken it but the exotic weapon proficiency feat was not required unless you have a RP reason for the wakazashi. Kukri has the same crit range, they do deal less damage at a d4 though. The base weapon damage won't make up much of your damage in the long run though and it would allow you to dedicate your first feat to something more useful. You might ask your DM if he would allow you to use the downtime retraining rules located here.
Prerequisite Combat Expertise, INT 13
Depending on your team you might also consider this feat as it allows your character to give up his crit to deal normal damage but the next ally to hit with a physical attack auto crits. So your big barbarian friend can deal massive amounts of damage if he has a low crit range weapon but a high crit modifier.
Prerequisite: Human
Following the theme of using your high crit chance this feat gives you a +2 on a single attack roll, saving throw, skill check, or ability check of your choice before the end of your next turn if you crit or roll a 20 on a saving throw.
Equipment Enchantment
As you are after damage increase I will be focusing on magic weapon enchantments.
Magic Weapon page for reference.
Speed is a +3 enchant.
Speed is one of the more common enchants that players use. It grants an extra attack for each weapon it is on. So as your character dual wields that would be an additional 2 attacks at the character's highest BAB each round.
Element Burst
Again following the Crit theme the element burst enchants. Each is a +2 enchant
There are four of these types Flaming, Icy, Corrosive, and Shocking each of them auto adds 1d6 of their element to all of your attacks. And when you crit they deal an additional 1d10 damage. The cool thing about these enchants is they can stack! So you can have a Flaming Burst, Corrosive Burst weapon that deals normal damage +1d6 fire +1d6 acid on normal strikes and when you crit it becomes Normal crit damage +1d6 fire +1d6 acid +1d10 Fire +1d10 Acid.
On the feats you suggest:
Fey Foundling—not really worth a feat for you. It would boost your lay on hands self-heal a fair amount (averaging 2¾ HP/level instead of 1¾ HP/level), which could be worth it for a character hyper-focused on self-sufficiency. But it does nothing for your damage, your protection from non-HP threats, your protection from one-shot kills, or your ability to heal allies, and it comes with a drawback (albeit a minor one). You can do a lot better than this in a feat, and you need feats way too badly.
Greater Mercy—terrible feat, at least in the long term. An extra 1d6 HP healed is nothing past the earliest levels. At best you can take it at 7th, when it represents a 25% improvement in your lay on hands healing, which sounds good until you remember that your healing is already rather low. 25% of a little still isn’t a lot. And the proportional benefit drops fast.
Power Attack—not amazing, but it may still be the best you’ve got. Only use this when you have both hands on your weapon; the accuracy penalty isn’t worth the damage unless you’re getting the two-handed bonus.
Extra Channel—you should not need this, you should have high Charisma. I would only get this if, after several levels, you determined you really really needed it on a daily basis. Remember, you still want a wand of cure light wounds, so you should not be burning through your channeling that fast.
Much more valid option if your GM allows you to retrain. Getting it early (when you are getting little from the half-level component and you cannot afford to boost Charisma yet) could be worthwhile, and then you retrain it once you are better-situated.
Antagonize—if you say this will work reliably with your GM, I cannot gainsay that. But if you aren’t looking to be the party face, that’s a lot of skill points devoted to making this work, and it will fail against a lot of targets, and it’s costly to use in combat. It’s the right idea for you, but I worry about it being effective enough to serve its purpose.
Weapon Focus—the only reason to ever take this feat is as a prerequisite to something else. Even then, you can just buy it for 2,000 gp, with a cracked opalescent white pyramid ioun stone and a wayfinder. Note that the cracked opalescent white pyramid ioun stone also gives you the ability to use some weapon as if it were martial, so feel free to grab a decent exotic if you can actually find one (you probably can’t, exotic weapons in Pathfinder are universally awful).
Selective Channeling—you should not be channeling energy in combat. It just doesn’t do enough. Kill the things you don’t want to heal first, then start healing. If channeling energy was an effective choice in combat, this would be a good feat, but it’s not so this isn’t.
Step Up—finally, a truly good feat. Yes, get this. Get this as soon as possible, do not wait until 15th level to get it.
Cleave—this feat is absolutely awful. You need to be full-attacking, not wasting a standard action to attack once. If you find yourself forced to move and thus left with only a standard action, cast a spell.
Great Cleave—see Cleave.
Improved Critical—happen too rarely to invest in, you have no special bonuses triggering off of criticals, and you probably are not going to be using a high-threat-range weapon. Why would you take this?
Angelic Blood—useless, but...
Angel Wings—this is fantastic. Angelic Blood is not worthwhile on its own but Angel Wings may very well be worth two feats. You need to get into the air somehow.
My suggestions:
Power Attack—it’s the basic damage boost. Not great but we’re only just getting started. Note this basically requires you to use a two-handed weapon, and is thus not necessarily compatible with every other option here.
- Furious Focus—potentially massive improvement to your accuracy when using Power Attack.
Antagonize—if you say you can expect it to work reliably, the effect is good.
Step Up—this feat remains amazing.
Improved Bull Rush—not bad, as moving your foes away from your allies and putting yourself between them should go a long way towards protecting them. Usual problems with the combat maneuver math hurt here: at higher levels, CMB just doesn’t keep up with CMD and it becomes very difficult to succeed. Smite’s bonus to attack also applies to CMB, though, which does help.
- Greater Bull Rush—a small bonus to bull rushing, which doesn’t really fix the mathematical problems, but giving allies a free attack is quite nice.
Focused Discipline—this feat is mediocre without the stamina rules, since fear effects aren’t used that constantly, but you’ll always pass and if you have the stamina rules, you can use it without anyone fearing you. The bonuses are only decent, but there are slim pickings at this point. Still better than, say, Weapon Focus.
Angelic Blood—as noted, awful on its own, necessary prerequisite.
- Angel Wings—flight is absolutely crucial. You could use divine bond to get a flying mount; that would be cheaper and probably better, but mounts have a lot of shortcomings. If you want your own independent flight, two feats is a steep cost—but you may have a hard time doing better as a paladin, and whatever you do, you must get flight somehow. If this is how you do it, so be it.
Divine Fighting Technique—without knowing what gods you have available, it’s impossible to say which of these you can take. The big ones to look for are Desna’s (which the SRD calls Way of the Shooting Star), which allows you to use Charisma for attack and damage with starknives, and Gorum’s (which the SRD calls Greatsword Battler), which allows you to use Vital Strike at the end of a charge. Note that neither of these would be allowed to a paladin in Golarion, as these gods are not LG and the feat requires that you match the deity’s alignment.
If you take Desna’s starknife ability, the Charisma to your attack stat does not stack with smite. This is antisynergistic, because instead of getting Str+Cha to attack against a smitten target, you get only Cha. However, that makes for a great reason to use the sacred shield archetype: your smite isn’t as good for you anyway, so bastion of good becomes a much better deal. Note that this also affects combat maneuvers, as combat maneuvers are attack rolls, which is nice. Starknives can also be thrown, which is well worth considering.
If you take Gorum’s greatsword charging ability, you will want charge feats (most importantly, Spirited Charge) as well as the Vital Strike line. These feats are ordinarily rather poor on their own, but by being able to use both at once, they become much, much better.
Dirty Fighting—I save this for last because some GMs may decide to houserule this away from paladins. That’s awful, though, because despite the name, there is nothing particularly dirty about this feat. Instead, what it is is a lot of versatility in one feat. It means that no combat maneuver provokes for you if you’re flanking, and it counts as having Combat Expertise, Improved Unarmed Strike, Dex 13, and Int 13 for prerequisites, helping you improve them. It gives you a lot of flexibility, and smite gives you bonuses to CMB for any maneuver. Unfortunately, this feat is not yet on the SRD, but only found in Dirty Tactics Toolbox.
Barroom Brawler—again, names that don’t sound paladin-y, but an effect that’s perfectly suitable. This allows you to get a combat feat temporarily, allowing you to meet a lot of situations you wouldn’t otherwise be prepared for. This is a very good feat.
Improved Trip—tripping is fairly effective, particularly at low levels. Definitely not worth getting Int and Dex in order to take this with Combat Expertise. Really, really wants a reach weapon.
Combat Reflexes—if you really want to trip, this is how you trip well. Requires a fair amount of Dexterity, though, even more than Following Step and Step Up and Strike.
- Greater Trip—requires even more AoOs, which means you’ll really need quite a bit of Dexterity to pull this off.
Improved Dirty Trick—Dirty trick is slightly “dirtier” than Dirty Fighting itself, but it’s also the best combat maneuver (just because of its versatility) and ultimately, nothing about it is any more underhanded that flanking, surprise rounds, and any number of other combat advantages that paladins are allowed and expected to enjoy. Definitely the best choice of combat maneuver if you go with starknives.
And something somewhat different:
- Point-Blank Shot, Precise Shot, Deadly Aim, Rapid Shot, Clustered Shots, Improved Precise Shot—these would largely be instead of most of the above feats. Could work with the starknife Divine Fighting Technique (definitely get a blinkback belt though), but most of the rest were melee-based. Archery, though, has a whole lot more feat-support, and deals a whole lot of damage even when you aren’t attacking a creature you’ve smitten. When you are, your accuracy and damage go way, way up.
Ultimately, the paladin has some fairly-solid defensive abilities in the form of divine grace and the auras; you do not need to spend feats on defense. Your out of combat healing is, similarly, quite adequate out of the box, and you do not need to spend feats on it. In-combat healing just isn’t effective enough, only useful in niche emergency situations. In those situations, just do what you have to do, but your feats need to be devoted to things you do all day, every day. Instead, my recommendations are devoted to eliminating threats as quickly as possible, and sticking to them in the interim to limit their ability to target your allies as much as you can. This matches your stated priorities, and is I think the most effective options you can take.
Unfortunately, as for order, that depends a lot on whether or not you can and want to use any of the Divine Fighting Techniques, and/or whether or not you can and want to leverage Dirty Fighting, and/or whether or not you are using the optional stamina point rules. Step Up is definitely something to take ASAP, and Power Attack and Furious Focus are similarly always-solid options. You say Antagonize will work for you, which helps a great deal. So those feats make the most sense as your top priorities no matter what.
But if you’re going with starknives, you want that ASAP because you want to dump Strength—which also means you’re probably not doing Power Attack. With Dirty Fighting and Barroom Brawler, you could do decently with combat maneuvers (aside from trip, which suffers so much without reach), particularly dirty tricks. Without, the archery line is almost-certainly your best bet there. Note that throwing allows you to combine both archery and TWF feats, but you probably won’t/shouldn’t have either the Dexterity or the feats for that.
Alternatively, if you can use the greatsword charging option, suddenly mounted feats and Vital Strike become high priorities, when they wouldn’t be taken at all otherwise. Most likely in that case you would want Power Attack, Furious Focus, Divine Fighting Technique, Vital Strike et al., Mounted Combat, Ride-by Attack, Spirited Charge, which will use up pretty much all your feats.
If doing neither, but allowed Dirty Fighting, getting a bit of Dexterity so you can trip as well as use the Step Up line seems in order. That will allow you to harass the opposition well and keep them off your allies. Remember that reach weapons are also two-handed, so you can and should still take Power Attack and Furious Focus. And Barroom Brawler is fantastic flexibility.
If stamina is in play, Focused Discipline is a quite-good feat for you no matter what you do. Without stamina, it’s lack-luster but still OK-ish. Fear happens enough.
And without any of those things, Power Attack, Furious Focus, Step Up, and Improved Bull Rush look best. Improvements to bull rush and Step Up follow those. Well, actually, at that point, archery is best, but besides that.
Best Answer
For Warpriest feats, I would recommend Vital strike over focusing on full attacks, you will likely be moving a lot of your turns with reduced movement from heavy equipment. That being said, Weapon of the Chosen is also good. Depending on the difficulty of encounters and your party's willingness to rest for extra days, you may want to consider Extra Channeling. Also, if you want to use Channel Positive Energy during combat, you will most likely need Selective Channeling so you don't heal your foes. Also, Warpriest levels count as fighter levels but only for the Bonus Feats gained at 3rd, 6th, 9th (etc) levels. It's indicated in the 2nd portion of the Bonus Feats descriptor on Paizo/PFSRD.
I wouldn't bother with Weapon Focus or Specialization, your class gives you plenty of bonuses in those aspects that they're not worth the feat slots. You only need Weapon Focus to unlock Sacred Weapon damage. (Assuming your deity's favored weapon isn't Katana)
For Blessings, Healing is very powerful defensively. It might also be wise to look at Knowledge and Liberation depending on your GM. War Blessing is always good in combat, but has less uses to a 'face of the party'.
Favored class you will likely be taking bonus health since it looks like you're the tankiest member of your group. I always take skill points, personally (even on fighters and the like). The Elf racial bonus likely wont fit since it limits which Blessings you could get bonus uses of (to mostly less useful Blessing choices).
In response to Ellesedil and your comment back, you could consider being a Bard if you only want healing spells. They use Charisma, have a huge number of skill points for Diplomacy, Bluff, etc and provide large bonuses to party defenses. There is no problem with using a Katana 2 handed and removing one hand to cast as necessary since it would be on separate rounds.
Remember with the bard, you don't have to be the court jester or a troubadour. You could be spreading the good word of the lord, but casting arcane spells. You would, however, lose access to medium and heavy armor.