Unfortunately, demanding that a person with a disability stop inconveniencing you when you are living in someone else's home by their favour is indeed disproportionate.
OP has already indicated she's happy to ask him to keep it down and that he is unable to do so.
So the DD's demands can't be fulfilled.
The only option the OP would then have would be to unfairly exclude her disabled son from her life and further favour the DD.
She chooses not to do that in her own home.
There seems to be a deliberately ableist determination to refuse to understand that yes she can ask him to keep it down and no he will not always remember to comply and that it is not him being unfair or unkind, it is part of his disability.
Once we accept that simple reality the situation becomes clear.
OP is also being asked to make disproportionate changes to her own life, she is to creep to her own toilet after 7pm at night and even the front door opening in her own home after 7pm is apparently problematic.
Her choice therefore is between an inconvenience for her daughter (the baby is absolutely fine of course) or excluding and ostracising her son.
She has chosen not to exclude and ostracise her son, who she loves, because he has a disability that DD finds annoying, partly because given his disability this will damage their relationship and cause him distress.
So she can simply continue to allow her disabled son to visit a couple of times a week as they have happily done until DD started making demands that cannot be met.
The baby is fine, nobody lives in a cone of silence, it's not possible. If DD moves she will not find a silent home after 7pm anywhere else either, or if she can find that she should simply go.
The DD is being done an enormous favour and will have to learn to cope with the nothingburger of occasional noise in the OPs home in exchange for a free and safe place to live.