OP - there are some overreactions on this thread I think.
Firstly - ignorance of breastfeeding doesn't mean they are bad grandparents or that they disrespect you or that they wouldn't be good people to care for your child.
I think there are a few different issues here - and you have asked for others experience, so I will tell you that I have had similar tensions over breastfeeding - simply because of my own feelings about it and lack of understanding in wider family.
the comment 'he can't still be hungry' is so common from parents in law and family - it was said to me recently and BURNED through my mind all night - I took it as upsetting and annoying, so much that while the in laws were here I went and hid in my room to feed the baby because I didn't want them to see me...my partner told me I was bonkers, that they were simply trying to be helpful.
Like many women I had absolutely no idea before I had this baby how time consuming breastfeeding is, I had not heard of cluster feeding and didn't know babies could feed so much - your inlaws clearly are the same as i was - ignorant.
That does not make them bad people! you know that yourself - deal with it one of two ways - either ignore it, or, do the grown up (but harder) thing of just talking openly to them - say 'actually he is still hungry, babies often cluster feed at certain times of day, its something we learnt in breastfeeding classes etc etc..or 'yes the baby is hungry, breastfed babies feed more often than bottle because their stomachs are so small and breastmilk doesnt last as long..'
thats how simple it is, once you say that if they say it again - then ignore them or get your partner to have a word.
the issue of their arriving in the hospital is horrendous, - your partner should have protected you from that frankly. and they were deeply insensitive - but again, it doesn't mean they can't be good grandparents, just be aware and stop it happening next time!
but I think people sometimes don't get how vulnerable women feel breastfeeding - I have had friends who have made me feel uncomfortable, and my in laws have sat and looked awkward while I struggled to latch the baby on - I wished so much they would talk amongst themselves but they probably didn't want to look like they were doing anything special, so ended up making it more awkward by keeping talking to me, which I hated.
if your MIL didn't breastfeed she may not know all of this - did she? Before I had the baby I honestly had no idea and look back with shame at how I sat in friends/ families living rooms as they breastfed their newborn, did I consider their feelings? no! I didn't even think it would be awkward for them.
I bet they are watching because they adore their grandchild and are fascinated to watch - they may really not understand how often babies need to feed. and perhaps the comments about you being up late are a bad joke - only you can know that, and I think that's worth you or your partner saying something about if they do it again.
give them a chance if you think they mean well.