You and your GF need to be an absolute team and your GF needs to know this.
10 weeks is a very short time and your GF is going to be hypervigilant for a long long time.
What's worse for you is your GF already doesn't get on with your mum, she' already sees your mum as interfering and the circumstances of the accident could be seen as your mum being careless about safety.
I appreciate your mum is very apologetic and guilt-ridden now and loves your son enormously. But given the previous history, what to your mum seems like wanting to make up for what she's done and make sure her grandson is getting better is just going to look like even more interfering. From your GF's perspective, why does your mum need to make sure he's getting better - that's her job!
So your mum needs to back right off. She needs to ensure she keeps her mouth buttoned about anything your GF does she thinks is a bit bonkers. She needs to obey all your GF's parenting rules even if she thinks a bit of custard or a sweet won't do any harm. If she can only visit at your house, when GF is there she needs make sure GF knows she understands perfectly why this is and she would have done exactly the same if it was her baby.
If she does all of this, then eventually GF might relax a bit. Again if this relaxation comes with rules - eg your mum's house needs to be childproofed all over, no kettles on display let alone switched on, whatever, she needs to go along with them without question.
If you and your GF start to argue over this, it could be a dealbreaker for her. Part of being a couple is that you put your parents second, and after you've had kids third. Your GF will need loads of time and reassurance.