parent transid verify failed on 2446869233664 wanted 20726247 found 20726245
This error is fatal, it means some writes were lost due to the device/kernel lying they were already done, this is usually a controller/device firmware problem.
If there's important data in the pool there are some recovery options here.
Legacy boot is always enabled, EFI or EFI- only allows or not to boot UEFI, if the the USB tool is not working try creating the flash drive manually, but it could be a board/flash drive problem or config issue.
That is normal, only used capacity is scrubbed.
Corruption in multiple devices is usually the result of a RAM issue, try with just one DIMM, if the same try with just the other one.
Avoid reading from the array until the parity sync finishes, it will slow down both operations, also might be a good idea to do this:
https://forums.unraid.net/topic/130825-help-with-partial-os-freeze-on-version-6111/?do=findComment&comment=1190253