I am so sorry your mum is not listening to your wishes regarding your son. I have a really overbearing mother but she would never pull stunts like this! It's sounds like as well as being overbearing she is also emotionally immature in not understanding you need a new car seat for your little one! Totally not over reacting! I wouldn't allow my mum alone with my little ones if she did things like this!
It sounds so toxic and there’s clearly lots more to it with how she treated you during pregnancy and post partum. Honestly, it sounds like you would be better without her causing the stress and upset in your life xx
I am really sorry that your mum is making you feel this way. It must be really tough. You are doing absolutely the right thing by putting your little ones safety first. Please don't let your mum make you feel guilty about doing that. X
Sounds toxic. Wouldn't put up with that kind of behaviour from any of my family or friends x