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 a relationship for each period. See e.g., https://musicbrainz.org/artist/2f569e60-0a1b-4fb9-95a4-3dc1525d1aad/relationships where Kevin Richardson left the group in 2006 and rejoined in 2012.
Add one relationship for “support member” that ends the same time the “(regular) member” begins. See e.g., https://musicbrainz.org/artist/79c67eb5-3851-4b28-bfe0-3adfcd6f58d7/relationships 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.)