That is such a good update. And frankly, from all the stuff you've told us, I would say your SD's description of your MIL is pretty much spot on.
One thing I tell my kids when people are being mean and hurtful to them is this: Remember, you only have to go to school with this person. Imagine the horror of waking up every day and being them.
That's what I thought about your MIL too. What a sad, horrid, bitter life you must be leading if you go to such extents to harm other people.