I know in CA, you're allowed to hire an aid through your own search and then the state pays them, so you're not just limited to those already employed by an agency. In this economy, people that would not normally be open to this type of work, might be thrilled for the opportunity. Your state rules might not allow for this, though...

I think your son acting as a buffer is a great final test. The fact that she's the lesser of evils that you've dealt with, is more than a little scary.

Oh--one last hint--headphones and an audio book or calming music, or even ear plugs and a locked door, with a note "Too ill to see you today, please do not disturb." If she won't respect your wishes in your own home...