My MIL and I were as close as mother and daughter for years. What happened?

You don't mention if she has dementia or what role her son is playing in her care.

I had to trick my (stage 5 Alzheimer's) mother into leaving her apt. to "go to the doctor for a routine checkup". I stayed with her the first night in the memory care facility and she was medicated fairly well. When I came back to see her in 2 weeks (requested by the staff=time to have her "settle in"), she was angry and physically combative to me. Even though she knew there was something wrong with her mind, she was fiercely independent having lived alone for 30 years after my step dad died. It took a few months for her to settle down and accept living there. During this time, she progressed from stage 5 to stage 6. Now she says things like she wants to go back home or they hate her and she wants to move but when I've brought up other places with new caregivers, she quickly changes her story.

Let her be angry (think about it-her home, her cigarettes and her neighbors are all gone). I'd be ticked off too. Probably the worst is having to concede that you are no longer able to care for yourself and what a huge blow to your self esteem that would be. Try to point out the things that she can do and praise her about them. She'll calm down in a while-it just seems like that will take FOREVER!! :(