You posted as I was preparing the following instructions. I'll leave them—for the record. Were you able to fix all your tags using a "Replace" Action in MP3Tag?
Case 2 is the one that works. UNSYNCEDLYRICS is the physical tag MC maps to [Lyrics]. If it has a language token, it has to be in the form "eng||" and not "eng|".
If your files do have lyrics in an UNSYNCED LYRICS tag, this is what you should do:
- Use Options>Library & Folders>Manage Library Fields>Add New Field to add the custom field [UNSYNCED LYRICS] (case doesn't matter, but it otherwise must be exact). Set Edit type to "Large Value" and set "Store in file tags."
- Add both [Lyrics] and [UNSYNCED LYRICS] to your view (a Panes View would be best for this).
- Select some files (try on a few before doing many or all) and from the context menu select Library Tools>Update Library (from tags) to read the lyrics tag to [UNSYNCED LYRICS].
- To copy [UNSYNCED LYRICS] to [Lyrics], open the Tag window, select [Lyrics] and enter "=[UNSYNCED LYRICS]".
If all applications can read [Lyrics] (saved in the UNSYNCEDLYRICS file tag), then you'll probably want to delete the UNSYNCED LYRICS tag. To do so from MC, select some files, delete [UNSYNCED LYRICS] in the Tag window, and then do an Update Tags (from library). You could also remove the field using MP3Tag.