Eeep!, I think it is more because there is an indication that they don't want to have some events that accommodate Mark and Mary. I absolutely think it is reasonable to schedule things that they can't attend but OP has indicated that there is not much interest in some give and take.
I have to disagree. One of the main reasons this is such a problem is that all they do is take. We have already changed plans and pushed the events to their house on 3 separate occasions despite all the shushing and the cramped space. They have attended exactly 0 of the other events that weren't at their house and have complained about it.
Friendship is a give and take but they're not giving anything.
I'm going to be ducking her calls at least for the next few days and luckily work has been hectic with a new project so I'll at least have an excuse.
You said earlier that there wasn't really a lot of interest in a quiet evening of conversation form time to time. As far as the times in their apartment, it doesn't sound as if it was all that hushed, more like they were asking that it be and then were dismayed to have at least some outbursts. I think that if it isn't something you want to make an effort to do to keep the friendship going then that is fine but it does seem to indicate that it is not a high priority friendship - possibly on either side.
I don't know if you've experienced this, but people can be unreasonable about their shushing even during a quiet night of conversation. They'll shush for levels of volume that are not actually very loud, or (in one case in my own experience) start admonishing people for not instinctively knowing the acoustic quirks of the parents' house. You could speak in a normal tone in this
corner of the room, but they'd bite your head off if you used the same tone in that
corner of the room, because sound carried up the stairs better from there even though the stairs were nowhere nearby, and so on. And they didn't brief people beforehand--"btw, try to be quiet in these spots, it'll wake the kid"--they just let people blunder into talking in the wrong places and then got huffy.
Because really it had a lot to do with wanting to control their friends (more than it had to do with not waking the kid), which ended up coming out in a lot of other ways too and lost them the friendship of most of our group eventually.