Sister Coyote, were I you, I’d go into the discussion with your mother planning to ask her several questions.
Has Gramma ever apologized for the shit you listed in the OP?
When was the last time she and Gramma spent an afternoon together? Then have her describe how much “fun” it was. If Gramma is as poisonous as you’re implying I doubt your mother has had an afternoon with her mother in years without leaving with her stress levels up, her self-esteem battered, and probably biting back tears or angry words.
Then ask her why she believes she can live with this person?
One of the key points for any of rescue work is for the would-be rescuer to make sure that he or she can act without becoming part of the casualty. Unless your mother is 100% sure that she can deal with someone who’s going to be using what sounds like emotional blackmail and abuse to control her, this scenario is giving me the heebie-jeebies.
If I were you, I’d do a little research before talking to your mom, though. Find the names of a couple assisted living facilities near where your grandmother is, now. Find which ones are recommended, and suggest that your mother try to point your grandmother to one of those. Offer to help her inform Gramma that she’s not welcome to live in your mother’s house, but that she’ll help her find a new place to live.