Woooooah, that's bonkers.
Change the key code. Today. Get a secondary lock and don't give her the key. If you need to give her access for an emergency, you change it back to the old code just for that occasion and you make sure you are clear that they key is being loaned, not given. I'm certain she will guilt trip you on this, but if she cannot respect your boundaries voluntarily, they have to be enforced!
When she tries to criticise you, shut it down immediately. Stay calm. "Ok, thank you for your opinion. I don't want to discuss it further right now as we aren't going to agree. Let's talk about x (something to do with her, perhaps, any bait you think she might take)." If she keeps coming back to criticism, just stand up and say (calmly) "Ok, I can see you only want to talk about xyz today, so let's just leave this here and talk another time". She'll sulk. But that's her choice. You need to be firm and calm, and just not take any criticism. She'll soon drop it.
Keep all visits somewhere public e.g. park, cafe (obviously not right now, but generally) or at her house - that gives you the possibility of an exit. Just don't have her in your house. If she brings it up, just casually quote something negative she's said about your house and say "so I thought this would be more pleasant for both of us" or make a positive comment about her house and how nice it is for the kids to have a change of scene.
If she's really good for an EXTENDED period of time (not just a month) then you could start inviting her in again. In the meantime, every time she knocks at the door (remember, get the code changed!!!) just smile and very calmly tell her that it's not a good time, so a visit isn't possible right now. Suggest a time (soonish) that you could come to her or meet in some public place.
You CANNOT continue to give someone this much power over you. It'll make you miserable. It'll probably drive your DH totally bonkers too, I'm amazed if he hasn't said anything! My MIL used to do this sort of thing, and it was a definite bone of contention between me and DH. It'll annoy your kids too as they get older and pick up more of the vibe, which will damage their relationship with her.