In the nicest possible way, YABU. Everything in your OP is all "me me me" with no regard to anyone else, which is quite an immature and selfish stance to take.
You have lost a grandparent, which is a terribly hard thing to do especially if you are close, but your reaction is OTT. It's one thing to go for a couple of days when you found out and then perhaps another few days for the funeral etc but to want to stay for weeks?!
You are on maternity leave and so you have much more flexibility with your schedule than your DH has, you can't hold that against him. Where I work they will allow you time off for the death of a parent, grandparent, sibling, partner or child. They do not allow it for in-laws or any other family member, and in those cases you would have to take unpaid/annual leave for any visiting and funeral. With you being on maternity leave I am guessing he has already had some time off just after your DCs birth and now, with only his wage coming in, he is probably loath to take unpaid/AL that he may need in 6 months time when you/DC are sick and you need him to stay home from work for a couple of days or for any other reason.
You have chosen to visit your parents for an extended period of time despite the fact that your DC is only a couple of months old, it is hours away from your own home and your DH has other responsibilities in addition to supporting you. You have created this situation so you can't complain when your DH can't/won't drop everything instantly to be by your side 24/7, especially when you're "a few hours" away from home and - I also presume - from where he works and where his family are. You're also making it hard for your in-laws to spend any tim with their son and his child because I presume they don't live near your parents so unless your husband drives to get the baby, drives back to his parents for a visit and then drives back to your parents they get no time at all with their grandchild, and I doubt he can fit that in with work, seeing you and his child and also supporting his family through a bereavement too.
I also wonder, and I don't mean this horribly, if you're actually making life harder for your parents by being there? One of them has just lost their father and they not only have to deal with their own grief and everything that comes with dealing with a death (helping or dealing with arrangements depending on the other immediate family, any wills and legal things that need doing etc) they then have their "devastated" daughter coming to stay with a 2 month old baby and all the sleepless nights, feeds/bottles/sterilising, sleeping and general paraphanalia that comes with such tiny babies. They might be glad of the distraction of their new grandchild, but equally it might be another stressor they really don't need (but are hardly about to admit to their daughter) at the moment.