I really really feel your pain op.
it is not YOU. Please take this on board and remember it.
This is all about her.
First of all an acknowledgement: believe me, I have been in your shoes. And even though my Mil mostly behaves herself nowadays, I'd never fully trust her and have surface forgiven her, but the way she treated me when I had a newborn and was vulnerable really traumatised me (and I don't use that word often if ever )
People can't often see this subtle underhand behaviour as a big deal (that's the purposeful design by the perpetrator) but it is a cruel form of bullying. Just below the radar enough to make you have you questioning yourself.
I could write you a book, I really could. But for the purpose of keeping this about you and not about me I'll try and keep it brief.
It usually boils down to jealousy. Good old fashioned jealousy. These women are often immature and see a woman in their son's life as a threat. They need to keep their No1 crown , so will make sure you feel second best. There's often mother -son enmeshment (mine was a widow and saw it as her entitlement to enmesh her adult sons as her surrogate husband ) mil still talks to both men in a silly baby voice, with baby made up words. It's utterly embarrassing. But very normal to them all. She's very manipulative and emotionally stunted and repressed. I believe she is jealous of my ability to be loving, open and honest.
Passive aggression is the go to tactic. (As per your examples) They'll target your "weak" spots and will play on your positive personality traits (kindness, empathy, eagerness to please ) and exploit them for all they're worth.
These people are not just mother in laws. They're manipulative and passive aggressive, jealous women, that just so happens to be your mil. It's their character.
Start reading a book called "the chimp paradox" (thank me later 😅) it'll help you react and process her digs in a logical way and teach you how not to react emotionally. Because an emotional reaction is exactly what they want. They do the subtle background wind ups. You end up the emotional, dramatic, volatile one when you eventually snap - they end up a crocodile teared victim. With the son being their shoulder to cry on. Don't fall for the manipulation. Eventually you'll learn to leave her spite with her. If she doesn't get a reaction, it's no fun. It's all a big fun game to her. She can only play if there's 2 players.
Ignore all the digs (it'll be hard at first ) literally pretend you aren't listening or disinterested. E.g mil: "... John used to love my dinners. Not sure about what he thinks of this new stuff " you: (yawn) "oh, that clocks wrong on the wall by 5 minutes. I must change that ". Do it every time. It takes the power off her. She wants you eagerly listening to it, questioning yourself and to become paranoid. They also love the idea of you arguing with their son. (Then they cry to mummy hopefully).
Anyway, that's a start ! I'll say no more at present as this is a phenomenally long message!