Can we first talk about Mega Gengar itself? This thing is an absolute monster that was banned and then unbanned from ubers. It's even better in this meta, as 130 speed is fairly good and it destroys stall with the perish trapper set. Much like how goth is banned in tier shift, I think this guy is disgusting. It's good vs offense too with the all out attacking set, it has a stab SE attack for victini, and name one mon that isn't weak to one of the infinite coverage moves that it can carry. It is an extremely uncompetitive aspect of the MaM meta that deserves to be banned. In a tier filled with threats such as Arceus, Victini, and Lando-T, the ability to trap the opponents checks/counters to these pokemon and eliminate them is absolutely devastating, as without these checks/counters, it becomes extremely difficult to stop these threats. The fact that trappers such as Mega Gengar does this with high consistency just makes things worse.
However, the main reason that Gar needs to be banned is the fact that it makes the meta matchup reliant, something that players can't actually control, into a far more important factor than it should be. Gengar has so many coverage moves that can just smash through whatever you want gone. Cress, Lugia, Mew all weak to shadow ball. Pixi spam? Stopped cold by sludge wave. 130 speed outruns everything listed in the OP besides glalite users, but YOU TRAP WHAT YOU WANT. This means it is impossible to counter and is extremely difficult to account for- I'm using mega stones as items, not shed shell. Ban this thing now.
As for unbans, lando-i and greninja can use mega stones, since they need their abilities + life orb to be broken.