Is BM willing to be a back-up?
Carolina, famously needs a quarterback, & has straight up said they have no interest.
what other teams do you see as possibilities for him starting?
I don't trust anything any team says about anyone right now. Its misinformation season. Carolina could be being honest, they could just as likely be trying to lower the price, or make people think they are taking a QB at #6 in order to trade down with someone who wants to move up. Nobody really knows, and frankly, I'm not sure the Panthers even know, though that is more of a comment about that organization than anything else.
I think Mayfield would absolutely be open to being a backup for a year, just not in Cleveland. He (understandably) feels betrayed by them. I think he's mostly right to feel that way, after playing through injuries that would have landed many QBs on IR, he toughed it out, and his reward was being replaced. I don't think that animosity carries over to a different franchise.
I think any of Atlanta, Carolina, Detroit, Houston, New Orleans, NY Giants, Philadelphia, Pittsburgh, Seattle (Geno re-signing doesn't prevent anything) or Washington, could be willing to bring him in to at least compete for the starting job if they don't address the position in the draft.
That's 10 teams, and there are maybe 6 QBs in the draft, depending on how the NFL feels about Carson Strong's knee. So 4 teams who will likely be left out in the cold, and will see Mayfield and Garoppolo as options. Personally, I think Mayfield is a much more attractive option than Garoppolo, since he's younger, cheaper to acquire, and in my opinion, just plain better.
I could see a deal happening during the draft probably on late day 2 or day 3, depending on who goes and when. I think the Browns will also be motivated to get him off the books, either by accepting a far below value draft pick, or by essentially buying a draft pick, and agreeing to pay much of Mayfield's salary.