Forgiveness is overrated.
I wouldn't forgive that, and I don't care what anyone thinks about that either.
You are not being at all unreasonable. your MIL could have easily said no, sorry I'm busy. Or just plain old nope, and you could have made other plans or sorted something else out. Or at least had a back up plan. Changing her mind at the last minute is really cruel.
I would never have her in my life again. Ask your husband how forgiving he'd be if you had not been able to be there for him during a possible life threatening procedure because your parent had flaked out in the last minute, ask him how forgiving he'd be to have undergone the terrifying experience alone in hospital.
You don't have to forgive unforgivable behaviour, and usually it is women expected to 'forgive' well fuck that.
Be angry, be hurt, be upset, you are still grieving I went crazy after my miscarriage which resulted in complications and would have not reacted too kindly to the 'forgive' brigade at all had I had to go through the operation alone, I was sobbing as I was put under anaesthetic and was utterly crazed with grief for a long time afterwards.
Can you ask your GP for grief counselling OP, you sound like you went through a lot of loss in a very short time.
Tell your H he needs to accept your decision and conduct his relationship with his mother sensitively to you. You don't want her in your life right now, and if he pushes it, that might become a permanent decision.
I so so sorry for your loss, I know a little bit of the gnawing agony, I do hope you can get counselling and the support from your husband that you need.