I was thinking about #2 . . .
before I saw your post.
To start, let me preface my answers by saying that I don't think that ISIS is Baathist per se . . . I think what you have is an unholy alliance between the religious megalomania of the ISIS leadership coupled with the military/political experience, contacts and agenda of the old Baathist party in Iraq.
Regarding your questions specifically:
1. I don't know why ISIS wants to depose Assad, except perhaps that it's part and parcel of the religious megalomania I referred to.
2. As I said, I was wondering about that, and my sense of ISIS is that its leadership's self-confidence about their standing as religious leaders is such that it cannot tolerate any depiction of any competing idea in any form. That's also why they behead Christians and any Muslims who don't acknowledge allegiance to their so-called caliphate.
Now, that's assuming that I'm on the right track with the religious megalomania idea. If that's not accurate, then ISIS is really a secular Baathist movement that is using religion as a cover for, and atrocities and military power to effect, its extension and consolidation of military, political and administrative power in the Levant. That could then also explain their attempts to overthrow Assad: he simply is weak enough and geographically positioned to be both in their way and a target they think they can take. That said, note that their focus on deposing Assad has waned with their push into Iraq . . . so Assad may be the lesser of evils/conquest targets when they can instead stick it to Shias . . . .