I agree with everything you said except for the point about referendum - both of them had it. The question is whether it was recognized by the rest of the world or not.

As for returning it back to Ukraine, I don't think either will be to happy about it. You may not believe me, but since 2014 the paradigm actually shifted into more pro-russia. People from there won't be welcomed back due to the past internal conflict and there is no more support from Russia. So in the end they Donetsk ends up kinda hated by everyone for no reason.

But even if we assume that everything goes smoothly and there is not hate towards people themselves, what's then? Any "legitimate" referendum will be forcefully shut-down, since I think everyone learned the lesson from 2014 (Belarus would be a good example).