1-As i said, it really depends on many factors, paladins at the start (fresh server) wont have any plate + heal gear, so they pretty much play as support being a dispel bot, using blessings and throwing a heal here and there. Later in the game they get a lot of +heal gear/enchants, positioning them at the top of the "healing" chain since they can heal for a long time without going oom, and still do their support skills. At the start the only real HEALERS are pretty much priests, for horde this will be the case through all vanilla, while for ally it gets as i mentioned before.
2- Depending in what bg, AB or WSG, but generally in WSG you ll count in both sides with Druid,rogue for sure and then 3 healers (in alliance you can switch since paladins and priest both are good at it, while in horde its IMPERATIVE to have priests or you re going nowhere). Then the DPS lineup can go as you like, i personally take for WSG 2warrs 1 hunter 1mage and 1 shadowpriest/warlock. It really depends, at this point you ll be looking for GOOD players rather than a fixed setup tbh.
3-Same as before, you wont be using the same spec through all the game, at the start you ll lack heal/mana if you run in plate and armor/sta if you run in cloth gear to compensate the lack of heal. Something like this would be standard, http://db.vanillagaming.org/?talent#sVxuRgotVMu0Th
4.theorycrafting is never an exact science.