FYI- I have been at/seen/am aware of multiple barns in New England that have a “shared stall” situation. There are horses on night turnout and horses on day turnout and they swap, usually only a few horses because the barn is overfull or because Dobbin is black and gets too hot during the day, etc. But the stalls are fully mucked and cleaned in between including changing water. Honestly I think this is pretty common. Although, usually boarders are ASKED if they are ok with this, or it’s the BO, BM or trainers horses that are swapping out depending on the situation. I also know of several places that have horses on 24/7 turnout and do not have enough stalls for every horse. Horses on 24/7 turnout have a shed in their night paddocks and there are emergency plans if there is a big storm/hurricane. I know one place had their barn built so that several cross-tie areas off the aisle could easily be turned into stalls if necessary, and you can always put up panels and/or temp. stall in the indoor.
So the stall swapping in general doesn’t bother me but the way they are doing it does. They should be fully cleaning the stalls in between, full stop. I’d ask that either her stall be fully cleaned or switched so that she isn’t stall sharing because of the skin irritation she is getting from the damp bedding (you did say something about damp bedding didn’t you?).
It sounds like there are a number of care items that have changed since you moved in 3 years ago. And it sounds like the BM is not that interested in rectifying the situation. She sounds… Not that into it, anymore. It might be time to move regardless.