Hi NormieNorma,
I'm a bit confused by this to be honest. You mention: "I think he confuses having done very well for himself given his circumstances with objectively being well-adjusted, for natural reasons" and that he was lucky to marry in to a "normie" family.
No one is less-than and he is no less than anyone else, regardless of his upbringing. I think it's nice that you have a great family together, it's not luck, I assume you fell in love with him and him with you, I guess it's lucky you met?
As for your Mother in law, as Anvil mentioned, whatever she is she is still his Mother. The only way to control this is to not have her in your house, otherwise it's just asking for trouble.
You may have to insist she stay at a hotel and you can then control the visiting times?