Bunny:

1) Paladin tanks really rely on their mana to keep aggro. That said, they also have the passive skill to get back mana when healed (nerfed a bit in 2.1, though). As far as tanking is concerned, a warrior is usually a better tank, but a paladin can throw a heal or a res if needed.

The question you need to ask yourself is, "Does the versatility outweigh the deficiency of damage mitigation and/or aggro holding?"

2) Ask yourself the same question. Does having another hunter or warlock outweigh the fact that your pet has less damage mitigation and can't hold aggro as well as a warrior (or paladin or druid)?

3) A holy-specced priest will heal better and for longer than a holy-specced paladin, although if your healer takes aggro, the priest will fall much quicker.

"Do I want bigger, more efficient heals, or do I want a healer who can last longer when getting beat on?" It depends on your skill at identifying and managing threat as well as your play style.

It seems like you are debating about two classes here. You want a paladin or warrior to tank and a paladin or priest to heal. Just make a paladin and see how he fits into your group. If you find a great spot for him, then you're set. If you don't, then axe him and get the purer class.