Thipu, it sounds to me like you're dealing with two strong aspects of human nature (shared by people of all ages)---one of which is almost impossible to shut down, but the other is one that will, I think, serve your purpose well without you having to take any action.
The first is the universal desire to feel important, part of a select group, to feel in the know. It's why it's so hopeless to try and use logic or reason or facts to talk someone out of their pet conspiracy theory. If Joe is part of a group of 100 people who KNOW that the stoplight turns red when they're in a hurry because the government is tracking them.....all the "facts" I can offer him mean nothing. Why would he want to give up being part of a special group and become ordinary just like the rest of us?
Fortunately, it sounds like your MIL is with-it enough to respond well to another, really strong, aspect of human nature: the urge toward self-preservation. Look how quickly she responded when faced with the threat of her GDIL's loss of respect. She reigned in her behavior, at least where GDIL could see it. I think the same will hold true with caregivers of whatever race. Unless she really loses some of her mental faculties, she'll have enough of a sense of self-preservation to cool it in the presence of people who could
And if she can't, it will probably be because she can't
---she won't have the mental ability to stop herself---and, as PPs have said, caregivers are used to people in that situation.
I do agree with (Luci, was it?) (my memory!
), the PP who said that just because we're older, don't assume we don't want to be corrected.....but the further attitudes you've described, Thipu, sound less like getting facts wrong and more like an emotional stance you're not going to reason her out of.