Zy - Your rule 2 needs modifying the test should be if the Artist tag/field for the subsequent tracks CONTAINS the Artist from Track 1.
My post explained the current rules and the new rule proposed by BryanC, showing that it wouldn't break anything as you were saying before.
What you are now saying with "contains" is a new proposal, different from what Bryan said.
Plus, and I'd hate to have to write it, there is another scenario where the result the OP desires would not happen - and that is where Track 1 contains more than 1 Artist but the Artist for all the remaining tracks is only the one whose name appears 1st in the list of Artists for Track 1.
You still misunderstand the proposal. Bryan is not talking about the Artist of the first file, he's talking about matching the FIRST artist in EACH file. So if you have:
Track1 Artist = John; Michael
Track2 Artist = John
=> Since John is the first artist on EACH file, then Album Artist (auto) would be John. Currently it's set to "Multiple".
Your "Contains" idea is better conceptually, but at the cost of more computation for a small gain:
Track1 Artist = John; Michael; Susan
Track2 Artist = Michael
Track3 Artist = Carl; Louise; Michael
=> MC could figure out that Michael is the common element and set that as the Album Artist (Auto), but it would need to check each of the artist values to find out the pattern. What if there are 2 common artists in all tracks? What about one of the tracks having blank Artist? What if that track is track 1? There are many edge cases to consider.
I think Bryan's idea is a good addon for now.
The simplest way of preventing MC from Making any Decisions about what Album Artists and Album Artist(Auto) should contain is for the user to make sure the Album Artist field/tag is never left empty; something that requires no changes to MC
Right. See Rule 1.