So this is basically an answer in two parts: some general observations about clerics, melee, and so on, which seem relevant to you, and then specific suggestions for your character. I’ve tried to keep the first section brief; feel free

General Observations

Just some things I want to establish first.

Touch attacks are usually really easy

Granted, a −2 penalty from Strength is large, particularly since most people would have a +2 or +3 there, but nonetheless, touch attacks are usually very easy to hit – and they only get easier as you get more BAB, because touch ACs don’t really tend to go up much for most creatures.

So if all you were concerned about was touch attacks, I’d encourage you to just hold off; chances are you’ll be hitting them quite reliably.

Divine power exists

Just so you’re aware, divine power is a ridiculously powerful 4th-level cleric spell that gives you fighter-like BAB as well as a +6 bonus to Strength. That will solve almost all of your troubles. It only lasts 1 round/level, which is problematic, but there are tricks for getting it to last longer.

Your accuracy as a Str 12 character with full BAB will still be fairly mediocre, but it will be more than sufficient to consider touch attacks as good as guaranteed. Your attacks against armor will be sub-par but not embarrassingly so.

You never really need a melee character

3.5 doesn’t go in much for “roles” or even “having a balanced party.” Magic rules everything in 3.5. You look at your party, and realize it’s got two full-casters and a half-caster, and want to improve it? You don’t get a warrior, you don’t get a skillmonkey: you get another spellcaster, because the more magic you have, the better off you are.

Clerics, as it turns out, have some of the best spellcasting in the game. Top 5, easily. Druids, too. (The other three are archivists – who mostly use the cleric spell list anyway – artificers, and wizards.) With a cleric and a druid, you’re actually doing pretty well.

And if one of the full-casters is looking to do more melee, the druid is the far more natural (har.) fit. Once he gets Wild Shape at 5th, he could be a beast (har.) in combat. Natural Spell at 6th is basically what every druid ever should do.

On houserules

• Losing inflict doesn’t really matter because those spells are really bad. Inflict light wounds is relatively cheap for repairing undead, but those are evil under the usual rules, so it doesn’t really matter.

  • That said, other things might be lost that would be quite a bit more painful. How much this affects how your cleric should play depends on exactly what your DM is restricting.

  • It may not matter, but for what it’s worth I can tell you unequivocally that negative energy is not [Evil], and the inflict spells are perfectly acceptable options for good clerics.

• Bonuses don’t stack if they are of the same type. Magic weapon applies an enhancement bonus, the same as a +X weapon, so those normally would not stack; that is not a houserule. Bless does apply a morale bonus that would stack, but it’s a small bonus that isn’t worth your time anyway. I would only worry about this if a bard joins your party (as the morale bonuses from Inspire Courage can be a great deal more significant than bless).

  • By the way, because of how this works, the usual response is to make every weapon a +1 whatever special properties weapon, and then cast greater magic weapon on it. Greater magic weapon will not stack with the +1, but it will replace the +1 and let you enjoy a higher enhancement bonus alongside special weapon properties. This is a good thing; special weapon properties are usually better than enhancement bonuses anyway.

• Your wealth is not good. Characters are expected, per the Dungeon Master’s Guide, to be worth approximately 2,700 gold pieces by the time they hit 3rd level. Moreover, wealth is expected to be primarily in the form of useful items, not in gold pieces you cannot spend. D&D 3.5 responds very poorly to lower-than-expected wealth. This “houserule” (which it isn’t really; these are just guidelines in the DMG, but they generally should be followed) is really the one I would be concerned about.

  • But not as a cleric. Clerics don’t really care, because they can make their own magic. It’s the ranger who should be very worried about this, because rangers only get really pitiful magic and need magic items to shore that up.

Specific suggestions

Basically, I see two routes here for your character: pure spellcaster (the easier and more powerful option, honestly), or, if you really insist on melee, ruby knight vindicator (requires a supplement you may not have, requires adaptation to work for Corellon).

Pure Spellcaster

Melee clerics are usually Strength-based – their ability to use armor while spellcasting means they can safely ditch Dexterity, and Strength leads to better returns on melee. Dexterity-based melee requires more feats for less damage. You don’t have that option, since you have low Strength and high Dexterity, but that Dexterity is put to better use simply pumping your Initiative, and allowing you to cast spells before other people get to go.

Clerics have some of the best buffs in the game. You can make each of your allies count for that much more. They also have solid battlefield control, to prevent enemies from doing the things they’d like to be doing. Choose your spells with these in mind. Don’t worry about damage – the ranger can deal damage with impunity if you pump him up and eliminate enemies’ ability to threaten you.

Ruby Knight Vindicator

Normally, clerics are just about the best melee warriors in the game. Normally, they have persistent divine power on top of a naturally-high Strength score, and swing big, two-handed weapons. You don’t have that option.

You could take advantage of your high Dexterity score by taking Weapon Finesse, but your damage – still based on Strength – will be poor. The best way to use Weapon Finesse is by dual-wielding, but that will make it hard to cast spells and you don’t have any major sources of bonus weapon damage, like Sneak Attack, to make that worthwhile. Ultimately, considering just how powerful you can be by not doing this, it’s really just not worth the headache. It can be done but why bother?

I can think of one exception that would be effective, and a lot of fun. The ruby knight vindicator from Tome of Battle is an awesome and fun prestige class that will make you quite a lot better at melee. It also works out quite neatly for you, because it enables a strong Dex-based option. Things it has going for it:

• It progresses your spellcasting fairly well.

• It offers martial maneuvers, which are simply the best way to get better at melee.

  • It offers martial maneuvers from the Shadow Hand discipline. This is important because

    1. those are quite good,

    2. being in a Shadow Hand stance is a requirement for the excellent Shadow Blade feat, which adds Dex-to-damage with certain weapons.

Problems:

• It’s in Tome of Battle; I do not know if you have that book availble

  • If you don’t, I strongly recommend it. It is, far and away, the best-designed book Wizards published for 3.5. No other book comes even close in tightness of design.

• You have to qualify. That basically means taking a level of crusader

  • This isn’t that big a downside; crusader is awesome.

• As written, it requires that you worship Wee Jas. The official Adaptation section says you can “easily adapt it to crusaders devoted to almost any other deity,” but you’ll have to take that up with your DM.

If you were going this route, you would want to do Cleric 4/Crusader 1/Ruby Knight Vindicator 10. By taking the crusader level at 5th, your Initiator Level is 3 (it includes half your levels in other classes) – enough to take 2nd-level maneuvers. You get 5 non-stance maneuvers total, and 1 stance; you need 2 Devoted Spirit maneuvers, at least one of which is a stance.

Stance – iron guard’s glare or martial spirit

For stance, then, your options are iron guard’s glare and martial spirit – both are quite good. Iron guard’s glare gives your enemies a −4 penalty to attack your allies – incentivizing them to focus on you, and leave the more fragile party members alone. Martial spirit allows you to heal someone nearby for 2 HP every time you hit an enemy (has to be a real enemy; can’t play-hit an ally to heal up). Both are good options. Don’t get too hung up on it, because ideally at 6th level you’ll assume a Shadow Hand stance and never leave.

Maneuvers – mountain hammer and at least one Devoted Spirit

For non-stance maneuvers, you have a lot of options; you can just pick whatever looks good, and it almost certainly will be. One needs to be Devoted Spirit; crusader’s strike for healing or foehammer for big damage are probably your best bets. Aside from these, mountain hammer is an excellent maneuver that basically every initiator should take, and I rather like battle leader’s charge and tactical strike from White Raven. But really, you can take whatever sounds good to you; it will probably work.

Feats – Weapon Finesse, Shadow Blade are top priority

You should also see if you can retrain your feats. Combat Casting is weak (in a few levels you’ll basically never fail a defensive casting check) and Spell Penetration just doesn’t help that much, particularly if you want to get into melee.

If you’re meleeing, with your stats, you need Weapon Finesse. That comes first no matter what; without it, you are not a melee character, and should see above for how to be a Pure Spellcaster.

At 6th level, when you take your first level of ruby knight vindicator, you gain a stance and Shadow Hand is an option. That means you can take island of blades for awesome flanking, and then you qualify – right at 6th level – for Shadow Blade. Assuming you already have Weapon Finesse, this is your highest priority.

After these, you want Exotic Weapon Proficiency, and then Power Attack. Yes, Power Attack – if you want damage, this is how you get it, and you should have the accuracy to support it. Plus, you should be getting 2:1 returns, because...

Exotic weapon – Spiked Chain

That Exotic Weapon Proficiency feat is for the spiked chain – a two-handed finesse weapon associated with Shadow Hand. That list just hit every major point we needed. You get 2:1 returns from Power Attack, you get to use Dexterity for both attack and damage rolls. Plus it has continuous reach, which is just about the best property a weapon can have. In other words, this is an absolutely perfect weapon for you. Since it can trip, you should consider Combat Expertise, Improved Trip, and Knock-down later on.

Without retraining – Weapon Finesse and Shadow Blade ASAP

If you cannot retrain, take Weapon Finesse at 6th and Shadow Blade at 9th. You can take Exotic Weapon Proficiency at 12th and Power Attack at 15th if you want, but getting your Dexterity into play is your top priority.

Let's work out some of the math here; the details of your build and your DM are going to influence how these numbers work out. Some assumptions I am making based on what you have stated.

Assumptions

1. You said DM is letting you use flanking rules so I am assuming you are getting advantage on each attack so I am going to state that you are getting advantage on your rolls 75% of the time due to other skills and tactics you could be doing.
2. I am assuming that your attack stat at this point is a 20 in any of the builds. I am ignoring barbarians right now who can get more then a 20 in a stat also I am ignoring belts or other items that can raise your stat above a 20.
3. Finally we are going to ignore the Sun Blade because getting a hold of a specific magic item should not be a requirement for a build.
4. This also assumes that your DM lets you roll your X number of attacks and decide to apply the sneak attack to the critical attack. The once per term requirement is very vague on this and should be clarified.

The Math

My first thought is that the thief build is going to be really top heavy; you are trying to get as many dice rolls as possible because you want that natural 20 critical to double the number of dice you are rolling for damage. I did a little look at monsters vs stats and I am trying to figure out a good threshold for when a die roll will hit and I feel like 13 on the die should hit based on average AC of monsters at that level. So 13 for die roll, 2 for weapon bonus, 6 for proficiency and 5 for stat. Now one could argue these numbers one way or another but I am going to be using them as the base because I need something to compare apples to apples.

So 13 + 2 + 6 + 5 = 26 for target AC to hit or 35% chance to hit and a 5% chance to crit. I used http://andrewgelman.com/2014/07/12/dnd-5e-advantage-disadvantage-probability/ as a reference for determining advantage so that translates to a 63.9% chance to hit and a 9.8% to critical. That makes it so 54.1% of the time you are going to do normal damage and 9.8% time you are going to do double damage. I basically am saying 2/3 of the time you are going to hit with your attack, really anything that gives you 2 attacks is going to let you get a sneak attack in during that round so long as you are within 5 feet of your target. This is also a pretty high number, some could argue that 10 or above would be a more realistic number then you are talking almost an 80% chance to hit with advantage.

Since you decided to dip into fighter instead of go 20 rogue you don't get Stroke of Luck but that would only be a factor depending on how much resting vs combat you would do. To ease math on calculating average damage I am assuming your weapon damage for rogue is a D6. If you are getting a D8 or better this just slightly makes your numbers better it just helps me to do my math faster.

Critical Hit - 18d6 + 2d6 + 5 + 2 = 77 Avg

Normal Hit - 9d6 + 1d6 + 5 + 2 = 42 Avg

GWF Critical Hit - 18d6 + 2d6 + 5 + 2 = 85 Avg

GWF Normal Hit - 9d6 + 1d6 + 5 + 2 = 46 Avg

The math was kind of surprising because of the limited reroll only on 1 or 2 and you could get a 1 after you rerolled a 2 but on average on 10d6 rolling a 1 or 2 and getting 4 extra damage on average.

Polearm Master

After looking at the polearm master the numbers are basically the same for attack and damage except that you get another attack that you will get your critical damage against. Even though the base dice is a d4 that extra attack is what gets you the key advantage — do you want that extra damage from the reroll or the chance to attack again and be able to sneak attack? Frankly I'd rather take the 63.9% to do my sneak attack dice again then the extra 4 damage on average.

Crossbow Expert

This basically follows the same rules as Polearm master: the feat is letting you get an extra attack with the a crossbow and I still think it is better then Polearm master because being able to attack at range is crucial; however, that depends on your flavor and what you like to do. However where this starts to break down is flanking and how your GM rules on flanking with a crossbow. If your GM rules you can't with a range weapon then you are stuck with polearm.

Bottom Line

Bottom line your GM house rule about flanking is getting you the most bang for your buck here because you want to maximize that rule. The winner between all 3 is really hard to decide, and also depends on how your GM house rules critical hits and determining which one was a sneak attack. If you get to pick the extra attack is going to be always worth it. If you can't pick which attack to apply sneak attack damage to then they are really almost a wash because your extra critical chance vs the on average higher damage from your rolling is going to end up being almost a wash in the long term at high level.