Between the fact that it's a good on paper fit and he's expected to have a narrow market, I doubt DeRozan would leave if traded, but if they're so concerned, an extension can be part of the negotiation (3/$75-81M probably get's it done).
Unless they can pull off a trade centered around Gordon for Russell, I don't buy this notion of them being unwilling to give him up. If they're interested, they have to know that's the logical centerpiece for the Spurs.
I'd call their bluff and take this to the dying minutes of the deadline, if need be. If they don't relent, I'd be willing to do Forbes and Carroll instead of Belinelli. If they still don't relent, then cave and accept Bamba, Fournier and whatever draft compensation can be mustered.
Bamba has struggled so far, but he's 20, has played 886 minutes in his career and has upside similar to that of Turner (Gobert was the talk when he was coming out, but that seems far fetched). He'd also possibly fill a long term positional need.