If no one else has it, then anything with Diplomacy is a great choice—get it with a trait if you don't have it as a class skill.
I don't think it is too much of a spoiler to say that the undead make an appearance in an adventure path full of words like "carrion" and "haunting," and that knowledge: religion will come in handy, as well as the ability to deal with incorporeal enemies.
Don't discount vanilla bard. Fighter+Magus+summoned monsters+bard is a lot of people who can benefit from inspire courage. Versatile Performance + Bardic Knowledge can really make your skill points go a long ways. (instead of maxing your knowledge skills, you can keep them at a number of ranks equal to half your level with bardic knowledge making up the rest).
Archaeologist is another archetype to consider if you want to get both bard and rogue in a single character.
An Inquisitor would be another interesting choice.
And the sorcerer is a solid choice as well. Make sure that at least some of your spells can affect the undead. It'd be no fun if all you take is sleep and color spray and encounter a zombie ;)
I don't have too much experience with rogues, but since you can sneak attack undead in pathfinder, it's not an unreasonable choice.
Anyways, basically, those are all good ideas—I'd say just pick the one where the character/RP concept you have seems the most exciting to you.
Carrion Crown isn't the most diplomacy-heavy module ever to see print, but since you're travelling with a trio of people who are likely to be pretty poor at it, I think that you're on-base with making sure that your character is something that will be decent at talking to people. Any of your four choices are pretty decent, and Beebs's suggestions are all good as well. Given the choice, I would probably lean towards Bard/Inquisitor, since it's nice to have a second person in the party with some level of healing competance, if only to act as a backup wand user.
I strongly agree on Bard or Inquisitor. If the Druid is gonna be in melee a bunch. I would lean towards Bard. If he is gonna be out of melee as much as possible. I would go with Inquisitor.
What makes the inquisitor such a good diplomat even with poor cha?
Inquisitors get a bunch of bonuses to social skills, and can take inquisitions (including Conversion, available to all of them) that sub Wis in for Cha/Int for other skills as well. They're naturally best at intimidate, but can be as good at Diplomacy as about anyone.
A summoner would work well
If you go Sorcerer, you can go with the undead bloodline. They have a pretty cool Bloodline Arcana...Sleep and color spray away.
Corporeal undead that were once humanoids are treated as humanoids for the purposes of determining which spells affect them.
I actually never liked the Dervish Dancer, but that's just me. Remember that as you level as a regular bard you won't be just Inspiring courage and that's it. At level 7 with a rod of quicken, you can do three buffs in a round. After which you can go nuts on archery or some such strategy with really good effectiveness. Your combat heavy friends will love you.
Ninja and rogue are cool. But don't play one unless you really enjoy positioning and tactics in combat. They are supreme opportunists but can be disappointing to those who just want to do damage. That being said, I love Rogues for the Knife Master Archetype, but Ninjas make good candidates for the Eldrich Heritage Feats, which can be really cool (Shadow bloodline, I'm looking at you!)