Yes, that works. If you press the key where äöü would be in a German game, it still writes äöü (at least in Fortnite). But you must have a Razer keyboard.
According to the official answer, yes, but we recommend asking this in a discussion, as experience has shown that you will get more answers there than with a single question.
Yes, it should, since this is a kit with 128 keys compatible with ISO and ANSI.
The right part uses 1u keycaps on the AKKO so you can use the same keycaps as you would on a Blackwidow v3 Mini.
This is effectively the following Razer item: https://www.razer.com/gaming-key... The caps should be compatible with most "full-sized" keyboards from this manufacturer. Unfortunately, we do not have an exact list.