That seems like a weird designation for the MCU earth, but okay. Earth 199999 it is. Couldn't have been earth 2008?
And I agree that those are the three most likely ways the mutants will be used, and that inclusion in earth 199999 is the most likely. I'd just like to see the X-Men on their own earth in the MCU multiverse; it has never sat well with me that mutants are hated and feared while guys like Daredevil, Luke Cage, Parker, and Thor get a free pass (at least as far as being superhuman goes). After Endgame, it shouldn't be hard to utilize universe jumping so we can still get the mutants interacting with the other heroes with little hassle.
As for Deadpool, I expect the third film to just do its thing, just like the previous two. Those films have never taken themselves or the wider continuity very seriously and I don't think the Disney buy-out is reason to fix what isn't broken.
One can hope. I've always been a little cold on Marvel Herc, but I thought their Ares was pretty fun, gave me Black Adam vibes in a good way.
"We all know the truth: more connects us than separates us. But in times of crisis the wise build bridges, while the foolish build barriers. We must find a way to look after one another, as if we were one single tribe."
~ Black Panther.
I've never totally bought the mutant hate as it's been shown either. It's a poor substitute for actual bigotry, and with more minority heroes joining the MCU, I'd rather hear about their experiences with the real issues. I wouldn't be bothered at all if the MCU didn't continue the mutant hate in movies
On another front, Echo is going to be introduced in Hawkeye. Whenever Moon Knight drops, that might be a way to introduce Kingpin and Daredevil. So, kind of a backdoor way to get some of the street level heroes in.