I'm so sorry that you are feeling pressured like this. I understand where you are coming from re MIL but I guess its your DH you need to explain too.

Do you have a good relationship with you Child Health Nurse, do you think DH could attend a visit with you and maybe having a professional explain that a baby is getting all nutrients etc would help. I know myself at times I would love to know just how much milk they do get from a feed and it is something that is difficult to get your head around.

I also think that there is a phamplet made for Grandparents specifically explaining the changes to how babies are fed today rather they how babies were fed 30 years ago.

Good Luck and stay strong, you are doing a great job.