I think the plan should be fairly straight-forward.
1. Trade a 2nd, or a player + a 3rd, for Jimmy and extend him to lower his cap hit. Draft Mond/Mills/Trask/etc in the 2nd (use the 4th's to trade up if need be)
2. If SF won't budge, trade a 4th for Minshew. Draft Mond/Mills/Trask/etc in the 2nd.
In either scenario - let Cam compete and show what he's got. Figure out what you want to do by camp. Downside is you've got 4 QBs you may want to carry. But Bill's done it before. I'd expect one guy to get cut though while red-shirting the rookie.