Now I want to see Nazi Mad Men.
With nukes, maybe at the time they set them up they were strictly under German control. But you need to be physically present to launch one, and as the empire grows, you need personnel.
John up to this point hasn't shown anything but total loyalty too. It's Helen that was the problem, and Himmler's fragile ego. He wasn't critical of John's leadership; he was whining that he didn't visit him in the hospital.
I also could roll with the fact that the Nazis are so arrogant that they didn't think having nukes in America would ever be a problem.
As to the deal John struck, the other younger guy was shown to be relentlessly ambitious. I can buy John was able to capitalize on it. He (the other guy) said enough times he didn't like being in America anyway. And John was taking on most of the risk in having to kill Himmler.