I don't recall any of the IE or SSI games making stealth party based. I always remember parking my party and sending in the stealth character or - in combat - hiding in shadows and backstabbing on a per-character basis.
I think it's logical to assume that a character learns as much (or more) from a failed quest as a successful one. Ideally I think there should be no truly "failed" quests, just different avenues of completion and story advancement.