Perhaps the solution is to not talk to her about the little one? Or whenever your MIL talk about things you are unhappy about and you can see the direction in which it is going, immediately switch the conversation? For example if she asks you did you starve your baby, maybe just simple answer he is eating well no details needed, and then change to maybe ask her if she is eating well instead for example. Normally a person who can read the room will notice that you keep changing topic and will realise that you are unhappy and will not continue with the rant.
My relatives are also like this, and I found my dad's response to be very useful. For example, when I was young, I was very thin, and they would ask if my mum starved me (my mum would be upset like you), but my dad's response would be to over exaggerate as a joke, for example like saying I ate so well like a princess, even better than him yet I can maintain but he grow so fat instead, then change the topic to how they never grow fat, issit because they are starved? He used different ways all the time, but basically the deal is to exaggerate without revealing any details (nothing about bowl is full or same as how your mil cook etc), as if telling a joke and then change topic to talk about something else or the other party instead. Usually this lightens the mood and gives the other party a 台阶 (step) to get down of, and you guys can leave it as that. Discuss other stuff later and don't move back into the convo about your little one to keep your peace.