1. I stole pretty much nothing and had more money than I knew what to do with for most of the game. That said, I invested heavily in crafting which allows you to to pretty much print money.
2. Leadership is great, you want a party member to max it. Bartering I took, but I never really tested to see how much things cost without. As said, I had more money than I knew what to do with. Charisma gets you a bit of bonus XP and makes some quests easier and more fun. I'd advise a few points into it on whichever of your characters has Pet Pal and make sure you always have that character be the one to talk to NPCs. Take Pet Pal.
3. Jahan is a very good crafter. Let him do your crafting. There's no reason not to.
4. Pickpocketing helps in a few quests, but I've no real comment on it. Sneaking is very good. Lockpicking is a lot more satisfying than breaking things open, and if you go the break things open route you need to repair your weapon a lot.
5. Never tried real stealth, always just went invisible when necesarry. No comment here.
