Valve employees have confirmed that the OLED display is incompatible with the LCD Deck’s Mobo. So you can’t upgrade just the screen.

It would probably involve needing to make a custom firmware for the controller in order to redirect it to a different server, and to reimplement its Wifi pairing.

Flatpak includes all the libraries Steam, proton, and games need to run, sandboxed and without having to mess with the ones installed more generally on your system. Can help prevent headaches and compatibility issues.

No. A lot of the stuff GamePass uses on Windows, like UWP, haven’t been reverse engineered yet. And it doesn’t seem like there’s much interest to do so, outside of the occasional GamePass subscriber that wants to use Linux like yourself. Cloud Streaming with GamePass Ultimate is your only real option currently. And at best you can maybe hope that Microsoft will partner with Steam in the future to integrate it there in some way, similar to EA Play.