Personally I would make a polite excuse (so not to lower yourself to MIL level) and keep your space from MIL. You have no idea of her agenda and right now you are not in a strong enough place within yourself to deal with her.
She could be genuine, but I doubt it after her outburst at the game. I would be looking for a serious apology for her behavior before I have anything to do with her again. It does not sound like she has even acknowledged it.
Question - do you want to go to h dads graveside this weekend? If the answer is yes, then go. If the answer is no, then say "thank you for asking but on this occasion I don't wish to attend". You owe your h nothing, he needs to know that he does not have you on tap to his convenience.
Please look after yourself and start putting yourself first, your h will just use you and abuse you if you don't.
