Oh Bacon. I've read this post so many times here (and written it myself) that it made me laugh, even though it's not at all funny.
First thing I would do is pay to have the locks changed. Nothing else will solve this problem. Then never ever loan her the new key for any reason, and do not leave it laying anywhere she can find it (or sneak it or steal it) to make another copy.
Because they truly have no boundaries and don't even understand boundaries, they have little awareness that what they are doing is snooping. When our son (my mother's golden child grandson) joked her that she was snooping (i.e. looking at all our bills and paperwork on one of her unannounced visits to our house), she got VERY OFFENDED: "I am NOT a snoop!!"

(as she was snooping).
My mother is also disruptive to my mental well-being. There's a number of us here on this forum who can relate. You have plenty of company. She is 88 and lives nearby, but I have moved to LC, which for me is 1X/week.
She says I don’t care about her even though I have spent tons of my time doing things for her and trying to make her happy. It’s never enough.
This is
classic.
My mother’s rage has me in knots for the last time.
What are you thinking of changing?