I've had a bit of experience of elderly people with dementia through FIL and friends parents. My main 'take' on it is that you can never start too early on putting systems into place. In some areas the wheels of the SS grind exceedingly slow and the care that this lady could need in six months may not be available because forms A, B and C haven't been completed and so on.
I think there's a difficult balance to be struck here between looking after your neighbour, not treading on her DD's toes and also looking after yourself, Robin. As your neighbour already is dependent on you to some extent then I think you are fully justified in trying to put resources into place now, because from what you say, the burden will otherwise fall on you. I'd try to get the DD to start the ball rolling but if she can't/is unwilling/in denial then you'll need to see what you can do, for everyone's sake.
I know SS in one area (and I assume everywhere) carry out an assessment of needs. They will take the person into a unit and watch them over for a week or two. It gives a much clearer picture than that of a ten minute home visit, although I expect they will need to do that, too. Once that is done, then appropriate care can be given. I think that assessment procedure is set in place by the Dr but I'm not 100% certain. She can be given an alarm for use, should she fall or have another episode when you are away.
Another person worth contacting could be this lady's Health Visitor. HV's look after elderly folk as well as babies and were great when my dad was ill and dying (although he had a physical illness, not dementia).
I hope you find some help with all this, Robin, it's quite an undertaking. Good luck.