I think Jack for Thornton would be OK. 2 million less per year, of course Jack has 2 more years on his deal, but he's not THAT old. At 30 right now, he should be effective throughout the remainder of that deal, especially if Malone can fix him. I would think he can, but it's hard to say for sure.
And Jack has shown the ability to play the 2 in the past. I don't think IT is going be on this team much longer, and if MT is indeed traded for Jack, Jack provides some sort of veteran backup for McLemore. He should get plenty of minutes at the 1 and 2 to justify the money he'll be making .. once again, provided Malone gets him to play better.
Jack can also brings us a player who could potentially start for the rest of the season if we deal IT away for something other than a PG in return. We literally do not have another PG on the roster who can play the kind of minutes IT does. Jack could be that stop gap for 30 games, and if Jack got 30 MPG with Cousins and Gay, I'm sure his numbers would look a lot better. It wouldn't be hard for us to return his value to what it was after last season.
Ideally Landry is the guy we deal, but if Cleveland won't do that and they offer us Jack for Thornton, I think I'd pull the trigger. Or ask for Jack and Miles expiring for Outlaw and Thornton.