1) You don't have to break doors or chests, there's always a key lying somewhere, hidden levers, etc.
2) It requires Pickpocketing and Invisibility (Aero skill). Outside Cyseal, Pickpocketing is worthless, so I recommend you to not invest any point in it. Why not invest these ability points in skills, wouldn't it be more fun (and more useful)? As the game progresses, you'll have many stacks of 99999 gold without pickpocketing and nothing to do with all the money. Bartering is better for money, since it will always work, on the other hand, you're only allowed to pickpocket once per person.
3) Sneaking in combat is useful defensively, and offensively on physical attackers with Guerrilla. If you want pickpocketing on a mage, you don't need to invest in sneaking, since you can use invisibility.
4) No, you can dismiss and reinvite recruited companions at will without negative consequences.
