As with most Sorcerers, your spell selection is going to have a far greater effect on your character's playstyle than feats ever will. Once your Luck feat requirements have been satisfied, consider filling the rest up with general, Sorcerer-useful feats, like Versatile Spellcaster (as Swooper suggested), Arcane Thesis (Player's Handbook 2) coupled with Metamagic feats, reserve feats (Complete Mage) and so on.
Unluck (Spell Compendium) and Alter Fortune (Player's Handbook 2) come to mind as luck-themed spells.
As for luck feats:
- Lucky Start is definitely the first Luck feat I'd take. Initiative is one of the few rolls that consistently matters for a Sorcerer.
- Magical Fortune is OK, I guess.
- Tempting Fate is kind of planning to fail, but it's likely to save your butt sometime. So sure, why not.
- Lucky Catch is far too situational to be worth it.
- I'd consider Unbelievable Luck or Survivor's Luck next.
Domain Access: Luck is a complete trap. Notice how many spells known it costs you. You're losing one spell per (presumably spell) level, and only get the meager benefit of being able to cast one domain spell per spell level per day in return as far as spells are concerned.
Regarding your question about Draconic Heritage: If you're talking about the feat presented in Races of the Dragon and Complete Arcane, it doesn't add any spells known to your list. It's the Draconic Legacy feat that adds them.
A single-level dip in cleric can get you three things: the Inquisition domain with its +4 to dispel checks, Turn Undead that can power Divine Defiance, and the ability to activate a prayer bead of karma for a straight +4 bonus to CL.
Ur-priest does get you the last two things, but it’s difficult to enter and you need two levels instead of just one to get Rebuke Undead from it.
Anyway, Divine Defiance is a feat in Fiendish Codex II: Tyrants of the Nine Hells, and it lets you burn a Turn Undead usage to attempt a counterspell as an immediate action. Obviously valuable.
In general, the cloistered cleric variant is a great 1st level to take for a lot of people: you get 4×(6+Int) skills, much better than you would as a warlock, you get all of the above, another domain, Knowledge Devotion from Complete Champion, and you get identify as a 1st-level divine spell, i.e. you don’t need the arcane material component for it. The Undeath domain can get you Extra Turning if you want it, or you can use the other domain to get all manner of excellent things. Check out the Dipping Cleric Handbook for more ideas.
Divine Defiance requires “divine caster level 3rd,” which warlock doesn’t qualify for. The solution to that is Practiced Spellcaster (cleric) from Complete Mage; that will get your cleric caster level (but not any other aspect of cleric spellcasting) up to 5th to meet the requirement. You will also most likely want Practiced Spellcaster (warlock) to get back the CL lost when dipping cleric, as that would otherwise hurt your dispel checks.
The eldritch disciple prestige class from Complete Mage could progress your cleric spellcasting along with your warlock invocations, but it’s not that great and it would require you to take another two levels of cleric to get the 2nd-level divine spells it requires. Not worth it. Shenanigans can get you in without those levels of cleric, and that might be worth it, but if you want to go that route I recommend just asking your DM to adjust the class’s requirements to begin with, rather than get into shenanigans. For example, eldritch disciple expects a Cleric 3/Warlock 1 entry, and then at 1st level gives you invocations only (so you are effectively a Cleric 3/Warlock 2 at 5th level) and from then on progresses both. Maybe your DM would be willing to flip it, allowing you to enter as a Cleric 1/Warlock 3 and then progressing spells only at 1st (so you are effectively Cleric 2/Warlock 3).
The other prestige class worth mentioning for any warlock is chameleon from Races of Destiny. The second-level bonus feat can be changed every day, which means it can be any invocation you need that day (by making it Extra Invocation), or if you get far enough, an Item Creation feat to go with your Imbue Item class feature. This makes Cloistered Cleric 1/Warlock 9/Chameleon 2 a pretty strong build for you, and when you hit Cleric 1/Warlock 12/Chameleon 2 at 15th, you’ll be exceptionally versatile.
I will mention, however, that I have done a counterspelling warlock quite similar to the one I describe above, and while it works more or less, it’s pretty boring. If you are really interested in a high-power spin on the idea of ruining other casters’ day, what you really want is a Black Tactica build: use the war weaver prestige class to buff all of your allies as a move action at the start of combat, and then spend your actions in combat readying to target anyone who starts casting a spell. You don’t ready a counterspell, however: you ready the most reliable single-target nuke you have. In addition to hitting the spelcaster hard, which is painful, this also forces a Concentration check with DC 10 + the damage dealt. It’s trivial to pump the damage well beyond what anyone can hope to hit with the skill check, which means they will lose the spell.
This is a very high-power build that may actually make your game less fun, as your enemies start using similar tactics and combat becomes something of a deadlock, everyone readying nukes against the others, and whoever dares to move first still not getting anything done.
Best Answer
Apart from the traits you listed, there are some more that help with Aid Another checks listed on d20srd.
Helpful is strictly better than Helpful (+4 vs. +3), with the downside of being raised by Halflings (via Adopted).
Fools for Friends from Second Darkness gives a straight +1 trait bonus to whatever your Aid Another boosts. It stacks with Helpful (since the latter does not grant any named bonuses), But not with any trait bonuses your Ally has, which there aren't many to AC.
Inheritor grants a +1 trait bonus to an adjacent ally, but you need to be a Gillman, and the same restrictions as above still apply.