@BlueLegume we have shared experiences on other threads, and apart from subtle differences much of your Cockroach Cafe stories sound like our M's read the same handbook.
I posted an update on my Narcisstic Mother thread yesterday which is probably worth reading. I am not going to pretend this gets easier. I thought I was doing really well after posting that original thread. Turns out I am not. We were in exactly the same position as your between September and the week before Christmas.
A few things to note:
Social services can't legally force someone into an assessment. You will need to wait for the crisis.
Agree with your sister now, how you want to handle any crisis and inform your brother. It is perfectly possible for all three of you to have different approaches. Also expect other people's views to change once you are in crisis (including your own). It is amazing how readily people accept the idea of caring, until it becomes a reality.
Expect pushback when you don't provide "perfect daughter" responses. This comes from family and the agencies and the pressure is excoriating.
Your M is going to be very vulnerable and it will be a side you haven't seen before. I am not sure how anyone balances the hurt of the past, with the compassion needed at this time. I am really struggling with this right now.
Crisis can happen very quickly and come out of the blue. I found the family members that should have been prepping for it, avoided the discussions as much as M had historically.
When it comes, it may be awful. Prepare yourself. Your list of "things that will trigger your concern" is good, but we found it really hard to discern what was just normal stuff vs not being well stuff. M is now in a position where everything is being treated as Dementia, but it probably isn't. For her sake I wish there was more nuance as some of her agitation is more likely MH than Dementia. But for me, it also feels like a whole new set of things to have gaslit back at me. "Well she was never like that before" "This behaviour is definitely Dementia (referring to her hoarding tendencies)." It is painful.
And keep posting. You are all keeping me sane right now. We can do the same for you.