I'm making a warlock build for playing through the OC/MotB, and am trying to decide whether to invest in bluff or diplomacy. Bluff is a class skill for warlocks so I could beat higher DCs if I went down that route, but it appears from reading online that in as per usual, diplomacy is - assuming equal skill levels - by far the best conversational skill. My charisma will obviously be high which would help with the cross class issues to some extent. I also plan to make the character LG*, so bluff is...not necessarily an appropriate choice for RP. My question is "Are the diplomacy DCs in the two campaigns low enough that having the cross class skill penalty won't make me fail too many checks if I have a good charisma, and is it worth changing from diplomacy to bluff to remove the cross class penalty?

*Yes, I know, alignment restrictions. I decided that since I wanted to do this character, I'd mod the game to remove the restriction for the warlock.

Edit: Oh, and a secondary question: within the OC/MotB, how useful is UMD? Looking on the wiki, it appears that the DCs get excessive rather quickly beyond a certain point. I'm also dipping into cleric and paladin for reasons of making the build more interesting, so I'd get the benefits of those classes without the skill.

Thanks for the advice. I'll still go with a warlock, though - I realised the disadvantages when I first looked at the class, and decided to try one anyway as a change from the conventional caster I usually play in PNP. I was not aware that the cap was permanently increased when you took a class skill, or that the able learner feat was implemented in the game. Unfortunately I'm kind of limited on feats and skill points already, so I'm not sure I can spare the resources needed to make use of those.

As long as you're modding stuff, you could consider making the Hellfire Warlock class available too for your alignment. It beefs up damage immensely for a very small investment, and you can protect against/compensate for the downside with potions of Lesser Restoration or the Death Ward spell. Or magic items that grant immunity to ability drain.

More importantly, it makes your character look -very- cool. Ramp up for combat: eyes start glowing, flames pop up around your fists, let loose. Style matters.

As for skillpoints: you could indeed save some by skipping UMD. Having a good social skill (or two) makes both campaigns much more fun to play, imo. There's little else you truly need. Concentration is the only must-have. Spellcraft and Lore are nice for RP reasons and give you more dialogue options, but power-wise you'll be fine without them. If you play a human with half-decent int you can get all of these, some other race can still get most.

UMD -is- useful, but mostly to equip monk boots. In a party-based game there's no great need to use wizard scrolls, crafting is best left to NPCs regardless, and other than monk boots there will only rarely be some class- or alignment-only item you need. Feel free to skip in favour of more diplomacy. I doubt you'll regret it.
(Though those +5 sun soul boots are -very- nice.)

According to all the documentation I can find, including the in game description, the Hellfire Warlock doesn't actually have an alignment prerequisite. Weird, but I guess the NWN2 designers and/or Wizards of the Coast assumed that since the player had warlock abilities, they must have met the warlock alignment requirements. I absolutely plan to take it, anyway.

I may rearrange my ability scores to get a higher int, so I can have more skill points. Does NWN2 have items which give an odd enhancement bonus to ability scores like NWN does, or does it use the pnp rules of +2/+4/+6?

Huh. Definitely weird. I never checked, I'll admit. But yeah. Go for it! Mostly the latter, though items go higher than +6 in MotB. (And there are some exceptions, like those +5 Sun Soul Boots. But not in the stuff you can craft.)
More importantly, almost none of them stack. In NWN1 everything stacked, in NWN2 things stack even less than in P&P.

High INT is usually worthwhile in NWN2. It too gives the odd extra dialogue option, which is nice. Warlocks aren't -too- stat-starved, depending on how you build them. And skills are fun to have.

Regarding social skills, its possible to have negative bluff and buff up to be more social with Gann, as seen here, search post for "Finch":

http://lilura1.blogspot.com.au/2015/04/recounting-mask-of-betrayer-part-iv.html