There are a lot of options for Schuster. Obviously 5/8th is the main choice, but he has played plenty of backrow as well and could be pretty devastating on an edge. He could slot in at 5/8th off the bench as you say; although, I've never been a fan of rotating halves during a game unless absolutely necessary.
I just cannot fathom Desmond leaving him out of the 17. Des may be stubborn at times, but he isn't stupid. He wants to win comps and he knows players like Schuster help win comps. He backed a young Foran and a youngish DCE back in the day and we have two premierships because of it. Schuster has even more potential and skill at the same age. I have been watching this kid for a few years now and his talent is immense. The other big advantage he has over other rookie halves is that he is a physical specimen and can certainly handle the physicality of first grade.