There is no solution. Your phone is gone. Should have never locked the bootloader while an unofficial ROM is installed.
That unless you can find someone who can unlock your phone and flash a stock ROM for a payment (black market solution).
Good luck.
Nothing. EOL models won't get any more ROMs. You can uninstall updates for the inject module app and disable it if you want to use root hacks for Play Integrity.
That's impossible. You probably tried the feature last time on Android 14. That issue exists in 2.0.108 as well and was only fixed in ROMs that are based on newer AOSP Android 15 base.
This is an official problem with specific Android 15 ROMs. It even affects AOSP builds.
As the error says, the required Activity is missing. We'll see if we can do something about it, but it's an issue with the original ROM.