2) Is there a "stop follow 'till end of combat" macro?:
nope


3) Channeling spells without holding down the button: How would you go about channeling drain mana/life with warlock while at the same time cast firebal or other "cast over time" with mage?
(and to complicate things: there's also the instant horion-spells and heals from the shamy while he's in close-combat to account for...)
there are 2 ways I can tell you. First is different hotkeys. Second is (I am not sure if it will work but it should do) to use castsequence macro. On warlock it is cast drain life and on 2nd button hit cast nothing. In the same time mage casts fireball on first hit, and casts fireball on 2nd hit. Something like that :P

/castsequence Drain Life,,

/castsequence Fireball, Fireball