[DaveBlake]

EDIT: In Picard, there is no "ALBUMARTISTS" tag, so should I simply edit the existing "ALBUMARTIST" tag with the above information, or create it as a custom tag? 

  
Like I said in my previous post add $setmulti(albumartists,%_albumartists%) to the Picard options and let it tag correctly for you.

No, do not change the ALBUMARTIST (note no s) tag, but you can manually add ALBUMARTISTS as a custom tag if you don't want to change the Picard settings. However then you will have to remember to do this whenever albums have multiple album artists.

I also found another artist with a similar problem, but in reverse. ie: Multiple MBIDs for "Artist ID", but only a single MBID for "Release Artist ID". Would the trick above work on that one too?

Different numbers of Musicbrainz Artist Ids and Musicbrainz release artist ids is fine, happens lots in classical music but less so in other genres.

Picard by default adds an ARTISTS (note the s) tag that identifies individual artists that may be combined with other words in the ARTIST tag so they can be matched to the mbids in the Musicbrainz Artist Id tag. It is just having multiple album artists and album artist mbids (called Musicbrainz release artist id is Picard), but not having an albumartists tag (with individual names) to decode them that is the issue.