This assumes it has already mega-evolved, which I may have mistakenly implied by calling it "M-Gengar". You're correct that if it has already mega evolved, Mamoswine can't switch out, but if that's the case, then I'm going to be looking at your remaining Pokémon and thinking about which of Mamoswine's attacks can maim whatever replaces M-Gengar. That's still a prediction battle that, if done repeatedly, may wind up costing you serious damage on every one of your Pokémon aside from M-Gengar. As your opponent, I'm alright with forcing you to rely on M-Gengar for taking out mid/lategame walls, as damage output against bulk is definitely not its strong suit.
Unfortunately, as the discussion is solely about m-gengar, I'm going to say the switch-in is mostly irrelevant as it is theory on what the rest of the team holds. The relevance to the rest of the team is for taking care of threats for a sweeper, as I previously stated that other speculation can't really help us further discussion. As a result, the damage that 'maims' is in question and prediction is also in question. However, there is little doubt that mamoswine could threaten mega gengar, maybe even force it out. In the future, though, speculating on the damage it causes to the 'team' is hard to argue with/against.
As I already pre-empted in my previous post, 1-for-1 isn't necessarily a win for M-Gengar. I'll reiterate: pro-ban players have argued that it "just" needs to take out one target and can then set up a sweep. This makes a faulty assumption about an unknown metagame that cannot be fleshed out without more thorough evaluation, which is essentially what this whole thread is debating, right?
It indeed makes an assumption. But just one target was not so much stated as just certain pokemon. Should it need to take out all 6 on the opposing team, it probably won't work. But Gar's role is still to select certain pokemon that can wall the sweeper (more so when building the team than in an actual game) and mold the moveset to eliminate those threats. And yes, it does indeed make assumptions about teams we do not know. Therefore, a better theory would be that mega gengar could take down x,y,z counters presented to the sweeper. We cannot determine the amount he can take but we can assume, fairly, he'll at least get one.
Absolutely not. I've already had a post dinged once for my own speculation into what will be OU and what won't, and I won't be baited into another round of that speculation by getting into a debate with you about whether the threats I propose are good enough to be OU in this generation. What you are requesting has been expressly stated as off-topic.
Perfectly content with this. Just remember that checks for gar exist, but counters must have pursuit. I think we're molding a definition of counter for gar as something that can kill him without getting killed when allowed to come in.
In advocating for a quick ban, the burden is on pro-ban posters to argue that (e.g.) only one strong Pursuit user could possibly exist on paper; otherwise we can only find out how the Gen 6 metagame shapes up through more thorough testing. If pro-ban posters cannot rule out all such Pokémon on paper - not just those who were OU in Gen 5 or those they subjectively think will be OU in Gen 6 - which is a difficult task in and of itself, then that is tantamount to admitting that we could all learn useful information from a suspect test. As far as I know, quick bans are only applicable in cases where suspect testing would be a "waste of time."
It really isn't a burden that we have to solve. If n+1 (where n=/= 0) pursuit trappers exist with the sole purpose of going after m-gengar, that is easily an argument for over centralization. An over centralized meta game, as seen in 4th gen, is reason enough for a ban (Aka garchomp).