I think what you're asking for are the kinds of philosophical questions that don't have clean logical answers. I agree that the cases of people advocating for groups they are not members of are more challenging, but I think they're also highly situation-dependent; for example what representation does group Z have already? And how do you quantify that?

In my experience a lot of these problems ultimately trace their cause back to some systemic imbalance that's often invisible to those on the 'winning' side of it. Take the idea of 'white privilege'; a lot of white people live in very poor or difficult circumstances, and probably don't feel privileged, but the real meaning of that phrase is that their situation would be even worse if they weren't white, and purely because of their appearance. So I think if you want proposals for dealing with these kinds of problem, then fixing the societal systems is where you want to look, rather than dealing with the proximate causes. Most of us can only work on the level of symptoms with this one though, hence my initial reply.