Thou... Shalt... Not... Lose... Caster Levels...
*shudders*
For more SP check out the Cloistered Cleric variant. By level 7 you have Divine Power, so the poor BAB is irrelevant.
The Nymph's Kiss feat will also be highly useful. Extra SP and bonus to all CHA checks (including turning checks) along with a handy bonus to saves? Yes please.
If allowed access to PHB2, Intimidating Strike might be worthwhile, especially if you can get the Pact domain (adds INtimidate, among other things, as a class skill).
- What combination of classes would suit such a character best?
Straight cleric, and perhaps any PrC that allows 100% spellcasting with no dropped caster levels.
- What cleric spells are best for improving fighting skills? For manipulating battlefield?
Combat Buffs: Divine Power, RIghteous Might, Divine Favor (perhaps quickened). Others as situation demands (Resist Elements isn't always appropriate, for example).
I'm also a big fan of all-day long buffs like Greater Magic Weapon and Magic Vestment. Frees up cash for other gear.
Battlefield Manipulation: Darkness 15', Silence, Monster Summoning (Dretch has stinking cloud, for example), Pro Evil (in regards to summoned/conjured critters). Consecrate, Hallow. Lots of higher level options (Forbiddance).
- Is there any way to get more skill points per level [besides raising intelligence or changing race]?
Cloistered Cleric, Nymph's Kiss. Church Inquisitor is a PrC from CD with strong(er) skills, especially social skills. Certain domains can add class skills. Trickery & Pact are great for socialites. Kobold for trap monkeys.
- Is it better to invest stat increases in strength or in wisdom?
For a self-buffing cleric, starting with 15 WIS and dropping each stat bump into it lets you cast 9th level spells "on time" without relying on magic items. Think of each bonus spell as another "stat buff" each day. So, WIS.
- Can War domain grant Weapon Focus in an exotic weapon [like bastard sword].
Sure, if your deity uses that weapon. But it can only grant martial weapon proficiency so you'll still be spending a feat on EWP. Usually not worth it anyways -- you'll want to go with THF and the biggest weapon you can use coupled with power attack.
- I was thinking about investing some skill points in the Use Magic Device skill. Is this skill useful?
If you're a CHA focused cleric, sure. There's nice synergy to be had from Knowledge Arcana, SPellcraft, Decipher Script, and UMD that all reinforce each other. Usually I just drop 2 points CC (for a +1 MOD) that allows me to make the check, and rely on things I'd have on hand anyways to boost turning checks (Eagle's Glory/Nymph's Kiss/Circlet of Persuasion) to help with UMD as well. UMD is more of a "last resort" out of combat thing for me though. I'd use it to fire off a scroll of invisibility to sneak around with Silence 15' radius, not to cast an in-combat spell.