As somebody with social anxiety, it sounds like your MIL also has social anxiety. Hence, the constant need to have to reinforce that other people can stand her. I had to train this out of myself in my late teens/early 20s.
I know it's annoying to deal with, but showing a little compassion and simply affirming that you're exhausted but her relationship with you is secure will go a long way for her. She'll probably get used to you being quiet around her too.
Please try August Uncommon Teas. August.la is the link. I raved about them to a friend who honestly became annoyed with me going on about them. Then, tried them. She called me and was like, "Oh, shit. Nico was right."