If it works fine, you can use it as well.
No, you can keep TWRP. I've removed TWRP flashing part because officially it's deprecated: https://topjohnwu.github.io/Magisk/install.html#custom-recovery.
Check and let us know.
As you are aware, new Stable releases (including rebuilds that are currently in the works) do not include our fix for passing SafetyNet/Play Integrity checks. There's the guide how to flash Magisk and relevant fix on these ROMs.
NOTE: You have to repeat the process (at least installing Magisk)...
It isn't a log from the newest version, it's a log from old DEV. It will not contain any information that could be needed to fix your problem.
Xiaomi didn't release ROM for Xiaomi 12 until now. As far as we know, they plan to do it in December.