I don't know that he has a clinical mental health issue per se, though you certainly never know, obviously anyone can suffer depression or anxiety. Either way, he could probably use therapy and he and the team could probably use marriage counseling (like that Metallica movie).
Sounds like vaguely positive news today that they met. And while it is entirely his own fault, it's also completely reasonable that he wouldn't be physically or mentally ready, just like a player might need extended spring training. You could also compare it to Haseley, even if it still feels more like a tantrum.
Still hard to know how sincere it all is, rather than just Klutch Sports conceding ground. But if he stays away for a month or the team can just get on with it (and like you said, give other people opportunities) and then may he can come back and help for a little bit knowing that a trade might come. Morey's talk aside I don't see how he can let this wait until the trade deadline or next off-season. And the other bit of news is a big reason why. Joel is always going to miss games and be injured over the next five years; someone has to be that other star to carry the load during those times.