I have spent a lot of my life around bossy boots, and all you can do is back off.
You have done your research, so the information is there if needed.
Make sure you stay in touch - phone as often as seems reasonable, within what would be a normal pattern (obviously more often than usual). Let her know that you are happy to offer such advice and help as she wishes. Don't try to tell her that things won't get back to normal - she will find out! If by any chance there is a little space when she appears to want advice, keep it for the most necessary thing!
Does she appear to want any practical help from you or her daughter? If so, be very careful about what you offer. I would suggest saying you can do nothing; but if say, staying for a few days would enable a conversation about who will help when you are gone, it might be useful.
My experience with people like this is that the more you suggest stuff, the more they play the 'yes, but..' game, and then can't back down. Better to sympathise that cleaners never do a job to such high standards (cross your fingers and give a mental apology to cleaners!) and wait for the penny to drop.
When it does, just be there to comfort her.
I'd also suggest that your niece tries to have a conversation with her dad about what he wants.