1.Not usually, but the main storyline "Legacy" and the terminal missions stay generally the same for all proffesions, except ent and trader who get NO legacy.
2.Yes, by going to a proffesion counselor. It's free the first time but generally gets more and more expensive until you're paying 6mill a change. You can find them in Mos Eisley, Theed, and Coronet... I think. ((Note:I wouldn't recommend doing this until at least lvl 30, that way your at the level where you have auto-gear that comes for whatever proff you choose.))
3.No healing potions in SWG, You use the "Heal" set of powers which you aquire at either lvl 6 or 8... I can't remember. Although since Jedi's have better defence ((I guess)) They don't get one till lvl 14.
4.Just look for groups doing PVP, ask if you can join in. and then go. ((Note:Restuss, big PVP zone, only lets you in at lvl 75 and up. Kashyyyk and Mustafar are alright but hard. (I wouldn't recommend Kash until lvl 50, and Musty until at least 80) But I'd avoid any pvp at all until lvl 90, you'll be fully charged and can get the best gear for your character))
Hope I could help!
(Going for my Helpful badge ;D)
MAIN Cridyskag:TestCenter, lvl 90 Jedi
ALT Papa:Flurry, lvl 50 Jedi
NOOB Jhedyn:Bria, lvl 28 Jedi