I can’t answer all of your questions, but maybe some.
An artistic director is usually a manager or some kind of “behind the scenes” person who figures out what repertoire a band/an orchestra is going to perform—ie., they’re responsible for planning the artistic direction of the group (or solist). This happens before the group ever plays the pieces and is not directly relevant on the stage.
A conductor is the person standing in front of an orchestra/choir and waving their hands or batons or in some other way conducting the live performance. This section should play louder now, that section should make an abrupt stop, those sections should slowly fade in, etc. It’s something that directly affects the performance on the stage.
Add one relationship for “support member” that ends the same time the “(regular) member” begins. See e.g., Pentatonix - Relationships - MusicBrainz where Matt Sallee was a support member since 2017, but in 2018 was announced as the permanent new bass singer. (Actually, seems there was some confusion with the edits. I think @TheFaR8 will re-add the support member relationship soon, so it may not be apparent the moment you look at it, but I don’t know of any other examples off hand.)
Supporting member is basically “not really a part of the band as such, but plays with them in their live tours” or whatnot. Same as “supporting musician” for a solo artist.
Sure, but an additional member is a member, while a supporting musician (we don’t use the “member” word there actually) is not So that’s the main difference.
Joe breaks his hand, I fill in for the tour until he returns - supporting musician? (But not called a supporting member. That was my error.)
Band X uses a core four for recording, and the same group of 10 for live shows - additional member?
But artistic director and conductor:
What do we call the band leader or musical director? Let’s take Joe Solo Artist. He’s on tour. He’s got a group of guys behind him. There’s always the one guy that everyone takes their cues from. Sometimes he is the only consistent person with Joe Solo Artist while everyone else constantly changes. Basically the right hand man of Joe Solo Artist. The one that makes sure everything musical is prepared before showtime, the one that makes sure it all goes right on stage. He’s the only band member that talks to the star.
Would that maybe be two separate listings? Musician and artistic director. Maybe even 3 listings, because he is the conductor as well, but not in the classical sense because he doesn’t use a baton.
A baton isn’t required to conduct a classical ensemble. It’s just a tool to make the gestures more visible to the ever further away musicians as the orchestra expanded. Quickly checking the history of it on Wikipedia, it sounds as if funnily enough it started post-1820, so actually a romantic orchestra would be conducted by baton — not a classical one
There is also a concertmaster, though for whatever reason (possibly no one has ever asked) we only have that as a recording/artist relationship, not an artist/artist one.
The reason for that one was that concertmaster is also by definition a member who plays violin. It might make more sense as an attribute of the member relationship, for artist-artist.
Tbh I never used it and can’t think of a case where I’d use it unless that is how the band describes the relationship themselves.
I do believe it is supposed to mean that it’s a bit less of a full member, but still more than a supporting artist. But I can also imagine that this attribute is sometimes wrongfully used as a not needed distinction to original- or homonymous-members.
Given what was said, I would not call a band leader an artistic director unless they also had that role. But I would call it a conductor. The guitar headstock substitutes as the baton.