1. As Long as your friend in in the same faction, you can party up with him.
2. Not without transferring the character.
3. Leveling is quite easy and is fun from 80 to 110
You could probably do it in about 1-2 weeks if you're dedicated.
4. With the changes with Legion, most healer classes have talents they can select to quest even in a healer spec.
Don't have the specific build handy, but the dps output is pretty good on my Holy Paladin, usually switch to ret though when I quest.
Also you can change specs for free now(in a city or inn), so I would suggest to simply switch to a DPS spec.
This post was edited by Hammerownz on Sep 8 2016 10:05am