Finally, A Mental Pigeonhole for My Mother-In-Law

 Carolyn Hax, summarizing in two sentences why my MIL is so threatened by my dear spouse's non-negotiable boundary drawing and independence:

"It requires two kinds of letting go: on the micro level, where you let go of goofy outfits as kids learn to dress themselves, or of messy kitchens as kids learn to feed themselves; and the macro level, where you accept that your child might not reach adulthood believing what you believe, valuing what you value or doing what you expect.

This is what an insecure parent fears most — and that fear is the root of controlling behavior."

Just exactly so.  He turned out different and different must be wrong.  This is exactly why we do NOT get along: I take instant and forceful exception to her controlling behavior, and my instinct is to smash it down twice as hard as necessary.  This is also why I don't visit them any more.  His parents, his problem.