If you restored an MC12 library all library field values should have been preserved. The auto-import tool can reread files and cause changes if the TPE2 fied is populated, but that should not happen unless the disk files have been changed.
Could you try to the restore your MC12 library again before doing anything else? Disable the auto-importer before restoring and check that it stayed disabled immediately after the restore.
If you want to preserve your existing MC13 library you can create a new library for testing this.
If the fields are correct after the resore you can update the file tags from library, but keep in mind that MC12 cannot read the new ID3v2 tag frames (some of them have changed).
If the library fields are incorrect we should investigate this further before doing anything hastily.
BTW, how did you tag the files as "various artists"? Did you use the Album Artist field in MC12?