John Smith would have never ascended to such a high rank
While it's plausible that a defeated US Military member could be swayed to join the ranks of the occupying victors because the benefits are clear for both sides, I find it highly unrealistic that Nazi high command would ever let that person ascend beyond a fairly low ranking post, for a number of reasons:
- He's clearly shown that his loyalties can change & could therefore never be fully trusted.
- He was of an adult age when he drank the Nazi kool-aid, and therefore could never be considered fully indoctrinated or intellectually galvanized to the cause, as evidenced in the case of Thomas, who was brainwashed from such a young age that he turned himself in, whereas John couldn't.
More likely, Nazi high command would have 'fire-walled' any ascension beyond a certain level as a matter of a safeguard policy, because they'd have considered the aforementioned points. And, history has shown the Nazis were experts in picking the "right" people for the "right" jobs.