Level 12 Analysis
Your AC is very low. At level 12, AC 18 is essentially a death sentence. I'm going to assume you have a source of Mage Armour from a party spellcaster, giving you an AC of 22, and a further Ring of Protection or Amulet of Natural Armour or Barkskin or something to up that by +2 to 24. 24 AC is still incredibly low.
The lowest attack bonus I found with a quick peruse of CR 12 monsters is +18, the secondary Bite of an Athach. That hits you on a roll of 6. The most common number I found was +23, which would hit you on a 2. I saw lots of +26, +28 and even a +33. Those all hit you on everything but a natural 1.
In other words, your AC is too low if anything targets you for you to survive. The DM will have to pull punches not to kill you.
Valiant Stand improves this. By not by enough. Enemies are still going to autohit you. Best case is +4 stat boosters on dex and wis, mage armour, nat armour/deflection bonuses, and valiant stand - 18 base, +4 from enhancement to stats, +4 armour, +2-+3 nat armour/def, +3 valiant stand = 32. That's a hit on a 9 from the lowest common attack bonus I found on CR 12 monsters.
Your Attack Bonus Is Pretty Decent, For a Monk. You have +16 to hit without Smite. With Amulet of Mighty Fists +2, that's +18ish to hit. With Belt of giant Strength +4 or Bull's Strength, that's +20 to hit. Everything that has AC that is supposed to be a defense has 26+, which is good - you hit most of the time. Power Attack and Flurry are counterbalanced by Valiant Stand, which you should be endeavouring to have always active by use of Stealth, which you don't have maxed out, uh, why? Popping up out of nowhere for a flatfooted smiting power attacking flurry should be your opening round action in every combat. Even your iteratives have a decent chance to hit, with this hit bonus. Only decent, though.
Your Damage Isn't Great. 1d10+1d4+5 (where is that errant +1 damage coming from?) averages out to 13 points of damage per strike. With Power Attack, that's 19 damage. With Smite, against evil or chaotic outsiders/dragons, you get /+18/, so 34 damage per strike, which is actually level 12-worthy. Too bad you get 3 total attacks per day with that damage. Sweeping Smite carries the Cha-to-hit bonus, but not the damage from the 'first hit'. It's kind of like a really terrible whirlwind attack that way.
Most of the time you'll be attacking for 19, on average. Some stuff is immune to bleed, or has DR/cold iron and good, etc, so it might be less. If you move, you only have one attack, so your first round barring stealth will nearly always be one attack at best. Rounds after that, you can flurry for four attacks, each which might hit for 19 damage.
So a max damage on an ideal round of around 34+19+19+19 damage, or 91 damage in 4 hits. That's not terrible. It relies on getting a full round action, though. It also relies on hitting with iteratives, DR of the kind you can get past, no miss chances etc. Most monsters at this level have at least 150hp, so you won't be winning in hp races. But you will be able to contribute a bit at least to the usual team of four on one beatdowns.
Your HP gets eaten in a round. With your AC, most CR9+ melee monsters are going to kill you to dead in a round. 11 Constitution on a melee combatant isn't a great idea, but then, you're a monk and need wis and and dex and str and also cha for being a paladin, so eh.
Your Saves are decent. They could be better, but they could also be worse. Most DCs seem to hover around DC 23 at this level, going up as high as 28 but also as low as 21. With your +11/+11/+13, you should have a 50% shot at most saving throws, and although fear and poison aren't exactly the most common life-threatening things to save against, it's still nice.
Power Level: Fighter.
This character can be relied upon to be roughly as good as an unoptimized fighter. The DM will have to pull some punches, and tough encounters (CR +2 or greater) will have a good chance of one-shotting this character by accident. In a party with Rogue or better power levels, this character will feel notably weaker than the rest of the group.
Note
Evaluating something without gear is hard, because at the lower end of the scale, gear can have more effect on your ultimate spread of numbers than the classes chosen. If your gear is random stuff, you will be weaker. If you plug it all into boosters for your FOUR necessary stats (dex, wis, str, cha), rings of deflection and other stuff that helps boost your melee stats to make you 'great' at that rather than 'mediocre to good', which includes buffs from friendly spellcasters, you will be stronger.
Note
This build is stronger at lower levels, where a handy Mage Armour plus your defensive stats can put you out of reach of opponents attack bonuses, and things are easier to hit and easier to kill. Level 12 is definitely where things are starting to go 'downhill', and they will likely get worse as enemy AC and AB increase and every enemy has flying or spells and abilities.
Note
The build has no movement capability other than skills-based climbing, swimming, jumping. I'm assuming again allied spellcasters will be patching that hole with buffs, especially since with the other gear i've assumed in your build you don't have the money to buy gear to get flight or teleport or whatnot.
Suggestions
Your main problem is that you're playing a monk. Without serious optimization, monks are very weak. You've done a bit of optimization here, and pushed it up to unoptimized fighter level, but fighter is still not great if you're planning to be in a party with casters. Or if you're fighting caster-style enemies like many demons are.
Most of the tricks I know to make melee characters more competitive are in 3.5e. I'm assuming there's no backward compatibility in your group. So that's all out the window.
Paladin isn't giving you much other than flavour, either. You've got a decent Flurry and Attack Bonus, but you're way too fragile to make good use of it, unless your enemies come in the 'always attack the guy using the full defense action with the tower shield' variety.
In this case, i'd recommend Summoner. Synthesist Summoner. It slices, it dices, it buffs, it has level appropriate abilities, natural attacks, the works. It is, in fact, so good that you could mix it with a Paladin dip and still be relatively awesome.
Evangelist lets you keep advancing as a Summoner while also adding a divine feel to the class. It's also the only prestige I can find that advances your eidolon aka living armour, so.
Other. than. that... there's a good rogue-based fear build, but I don't know how relevant that is to your concept.
I feel like there's some way, between Fighter archetypes and the Hungry Ghost Monk monk archetype, to create kama-wielder who reaps ki from his opponents and uses it to chain-stun them with a huge Wisdom and Dex, just whittles them away with flurries of weak, ki-reaping attacks and then spends the ki on stuns. There's too many damned archetypes though.
Even if you're taking summoner, it's kind of worth dipping monk as well. You have to be unarmoured to use your synthesism, so you could benefit from a high wisdom score to your AC. Not worth going Hungry Ghost, because the ki-drain only comes at level 5. Just a one, or at max 2-level dip. Probably after you have 1 or 3 levels of synth summoner.
Yeah I cannot find anything that is about punching people that isn't just a Fighter or Barbarian with a 2-hander sword that is any good. There's some rogue stuff that is nice, there's an alchemist archetype that gets sneak attack and would make a murderously effective hand to hand combatant with the mutagen and the buffs. But nothing based around jumping on things and punching people except synth summoner. Yeah. That's my advice.
Synthesist Summoner 20.
Optional; Monk 1.
Optional; Evangelist.
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.
Best Answer
Are you going to be in a party, particularly with a rogue or ninja type? You may want to consider upping your Int, taking Combat Expertise and Two Weapon Feint/Improve Two Weapon Feint. You can then sacrifice one 'swing' in your full attack to flat-foot your opponent until your next turn (including for allies and attack of op). CE also gives you significant bonus AC at a low cost on your to-hit.
You might also want to look into Swashbuckler Opportune Parry and Riposte. It allows you to ready attacks to make opposed rolls instead of letting them just hit your AC and can be better than fighting defensively.