I've been thinking about buying a Series S for a long time. Compatibility with older consoles (One / 360) makes it possible to play these games as well. Here is my question: Is it possible to redeem a game key that was originally for the One on the Series S and then play it in reverse?
As long as the key is for a game that is also supported by the Series S | X, it should work
Yes, of course you can. The Series consoles are fully downward compatible with the One.
Anything else would be pointless because of the few Series Exclusives.