Keep in mind that Coach Z might be reducing the amount of time the team spends together so that they can pay more attention to it personally. Their intentions are probably good, and solo mentoring is very hard, so don't try to attack them but definitely bring up the possibility of allowing the students to be more responsible for parts of the team.
If this doesn't work: who runs the club on paper? Who's most connected to the school / part of the faculty? Go to them, get things fixed, and
if you have to create constitution that doesn't allow for things like this to happen.
Sorry you're having problems with Coach Z.
