if that function is for signing custom efi binaries, you can still only sign it using your own keys. that would work to attest that you trust it and approved running that thing with secure boot enabled, but anticheats are looking specifically for the approval of microsoft, for which microsoft would need to sign it with their own key.

you probably wouldn’t get banned, the game just won’t launch. but be sure that you are not runnig the “process hacker” task manager along with the game, neither similar tools inname because formerly garbage anticheat was triggered to the “hacker” name in the process name. I don’t know if that would trigger a ban though, or they just wouldn’t allow connecting, but since this is niche software and they won’t have any recourse for false positives maybe I wouldn’t try my chances unless I wanted help in stopping playing that game forever.

No, Windows anti-cheats will check specifically for Microsoft’s keys in use.