I would like to know:
1. Do I really need to be the Complete Divine Favored Soul to take this?
No. The prerequisite is to have Energy Resistance as a class ability. Classes that have this ability include Champion of Gwynharwyf (BoED), Sentinel of Bharrai (BoED), Warlock (CArc) and some associated prestige classes (Eldritch Disciple, Enlightened Spirit), as well as Favored Soul. Potentially the Spellthief's "Steal Energy Resistance" would allow qualification if the DM thought the flavour of the feat was appropriate to the class, although strictly speaking the Spellthief doesn't have spell resistance as a class feature even if he has currently stolen some: he has the ability to steal it.
That said, some classes are better than others for the purposes of this feat, as the mechanic includes your
divine caster level for the purpose of measuring how much energy you deflect. Bearing in mind that you forgo your saving throw if you pull this trick, and that you automatically take any damage that wasn't deflected, it is Not A Good Idea to qualify with a class that has piddling resistance and no divine CL. Moreover, the feat only affects individually-targetted spells... which invariably do a lot more damage for their level than area effects. Unless you plan to go around perma-buffed against the energy type you're resistant to, I can see this feat being a really elaborate way to commit suicide against a blaster-optimised caster.
Oh yeah, and you can only deflect energy of a type you're resistant to in the first place, which means the feat is a big waste of everybody's time against everything else. This is why Favored Soul is the only class that's really going to consider doing this: they get resistance 10 against multiple energy types
and have a divine caster level that might make use of the feat marginally worthwhile.
2. Are there ways to condense enemy spells to only target you?
I'm not actually sure what you're asking here. If you mean "can you negate area effects for everyone other than you?", the answer is probably "no". If you mean "Is there any way to force a spellcaster to only attack you with their spells", the answer is "yes, but if you've successfully subverted an enemy caster to that extent you'd be better off getting him to
not attack anyone with his spells".
I refer, of course, to mind-affecting effects like
dominate person...
The Knight's (PHB II) Test of Mettle class feature could also potentially force an enemy spellcaster to target you, but there are a few problems:
1) "If a foe attacks by casting a spell or using a supernatural ability, he must
target you with the attack or include you in the effect's area."
As he's not restricted to attacking you
exclusively - or indeed
at all - but is instead forced to
include you in any attacking spells he does cast, the only way you could ensure that none of your allies were included in any multi-target or area effects is by being so far away from them that no spells could possibly include both you and your party. Good luck with that.
2) You have at least 4 levels of Knight, and if you want Test of Mettle to have any significant chance of affecting a spellcaster you actually care about, you'll have to take a lot more levels in it.
3) In the unlikely event that you succeed in getting the spellcaster's exclusive attention, anyone
else attacking said caster will end the Test of Mettle effect, making it pointless, and...
4) ...you'll die, because you have a class feature that can potentially force a caster to want you dead more than anyone else on the battlefield, but exactly zero class features to prevent this from actually happening.
3. Can characters with energy immunities still use this feat?
Yes, but only if they
also have energy
resistance as a class feature. The mechanic for deflection is based on the amount of energy you can resist. Energy immunity isn't "resistance to infinity", you're just immune to it. However, if you have energy resistance (from a class feature)
and you're immune to that energy type, you're good to go. You deflect energy back at the source and don't take any damage yourself... However, I still think there are generally better uses for both feats and immediate actions.
Now, a cunning player
might argue that a Cleric's spells (for example) are a class ability, and that if he casts
resist energy on himself he thus has "energy resistance from a class ability". And the same cunning player
might find themselves on the receiving end of sourcebook-inflicted nonlethal damage as a result. All things are possible
