We have been through something similar with my nan, who is now in a residential care home. (not just assisted living). Given her diagnosis, I'm surprised they recommended only assisted living for you nan 18mths ago. Especially if the facility doesn't also offer higher care in another section of it. Unfortunately, as I'm sure you are sadly aware, Parkinsons isn't curable and will no doubt get very much worse. This can happen gradually, or in progressively worsening steps, especially after an illness or fall. There is no excuse for taking an hour to answer a call bell, but in the assisted living places I'm aware of, they are only used in an emergency- not for someone needing your nans level of care.
My nan was living alone, in her own house, but also having falls despite having a commode. In the end, my mum was doing her washing, we'd cook meals for her, take her out, wash her and was there daily, which just became too much. How is your mum coping with the additional tasks she is doing for your nan?
Other than falls and having someone there overnight, a big part of moving her to a care home, was so that she would be familiar with the rooms and facility before her dementia got worse. Yes, the move was difficult for her, BUT, we are happy she is settled and has the care she will likely need in the coming months.
I'd recommend looking into a specialist belt she can wear which helps prevent hip fractures during a fall. I'm unsure if you need a GP,
occupational therapist or physio to advise, or whether you could just buy one? I too would speak with the manager where she is to see if the care being given, really is what that facility normally provides.