As I said a few weeks ago, I think the real question is: is he set on going back period, regardless of what team had or could trade for his rights? Or is he set on going back primarily because of the Sabres depth at LD?
I think, depending on who is his agent is and presuming that Adams has a good relationship with his agent/agency (there's always mutual benefit for agents to have good relationships with GMs from all teams), that hopefully Adams could get a truthful, candid answer to that question. Same way that Winnipeg got with Nathan Smith this year, and Minny got with Jack McBain, which allowed each of those teams to trade those prospects at the trade deadline.
If Adams can at least get indication that Johnson would be willing to sign now with one or more other teams, he could explore trading his rights at/before the draft. If we cannot get a straight answer on Johnson's intentions or interest in signing with the Sabres after this season, the Sabres should trade him at the draft.
Separately: I wonder from the player's perspective in the case of Johnson or guys in a similar position (like Lacombe and Faber) if part of the reason they want to go back to school is if they actually feel like spending another year in the NCAA may obviate the need for them to have AHL seasoning. It is a lot easier to make the jump directly from college to the NHL and bypass the AHL if you're another year older/more mature and have another year of high level college play under your belt. I don't think this will be the case with most players, but you can definitely see that the more talented/high end prospects may not need the kind of AHL seasoning if they play 3 or 4 years vs. if they played 1 or 2. So they get the benefit of extra development time while staying in school, and also retain that UFA option should they choose to pursue it. It's a smart strategy for the player, it just sucks for the team that drafted them if they don't end up signing.