I have same problem (the first one) with multiple language layouts for physical keyboards on S7 tablet. And it's Samsung OneUI problem that needs to be fixed ASAP.
The thing is that language selection menu in "Settings > General management > Language and input > Physical keyboards" does not support multi select and allows you to choose only one language. Same behavior for Samsung Keyboard and Gboard.
I tried same steps with same bluetooth keyboard on different device (on latest Android One) and it's working flawlessly. And that is only because then i go to "Language and input > Physical keyboards" and click on any keyboard - i see list of languages with multi select options.