Mi Video doesn't support Chromecast/Google Cast. It only supports Mi Link, which as far as we know is supported on the Chinese Mi Box (not Android TV).
Play Store: Settings -> Manage apps -> Google Play Store -> Clear data -> Clear all data.
Themes: Settings -> Manage apps -> Themes ->...
If you live in the EU, you can try to manually edit build.prop and change ro.miui.wifi.region from GB to your country code. That might prevent you from using 5GHz hotspot (or might not), but other than that you should be able to see all supported channels in your country.
We don't really care whether it worked for you on stable. Your device does not have a proper implementation of Camera2 API.
Forcing it to use Camera2 API will cause unexpected behavior, and we don't provide support for this kind of tinkering.
Something not working as you expect it to work is not...
That code doesn't necessarily enables VoLTE, it overrides operator VoLTE-support check, and forces it to show as supported in the settings screen.
It's a debugging/testing code and not meant to be used by end-users. We have a Hungarian user with actual VoLTE support, and it's available without...
Nope. Should not have bought a device with the dreaded notch.
And please, if you're using third-party mods, don't report bugs here at all.