Thing is, I could easily rattle off a dozen different variables that might reasonably make their way into an M rated game. I mean, how would this solution of yours manage things like drug use, prostitution, sexual violence and gambling?
I was discussing those four since three were brought up by the OP and the fourth brought up often enough to address.
For your four, three can be handled easily by merely making it optional content. Gambling even was such in Mass Effect 1. As for sexual violence are you talking about in the game period or can the protagonist do that? I doubt Bioware will do the latter, and the former would almost certainly have it shown to always be a negative and probably be a case of tell, not show like they do with similar things.
No, adding extra animations for a few people who want to play M rated game but don't want to play M rated game is a wasted resource when they could working on something else. Those extra animations would need to work for many enemies. We're not fighting one or two enemies only in the game.
Yes, because no Rated M game has ever had it so you can play a character who fights non-lethally.
Well, ignoring the most famous franchise that does exactly that and is hailed for it: the Deus Ex franchise.
Again, how do you know that? In fact, in every Mass Effect game we fight at most half a dozen different enemy factions, almost all of whom use the exact same animation skeleton.
When you skip something, you are purposely missing it. That's the point.
I mean like conversations. For example you didn't miss anything in the Shepard Trilogy by skipping the sex scenes other than the sex.