OP the more I read your posts the more sorry for you I feel, as you seem to be dealing with some vastly different people, who are likely to behave in vastly different but equally unreasonable ways.
1st Set of Parents: Had to be told because you were ill, you see them regularly, they would have worked it out. They can be trusted to respect your wishes. You and your husband are in agreement on this.
2nd Set of Parents: Will be told shortly, because you don't often see them in person and this will be your best opportunity. They are likely to take offence at being told last, and this will cause some issues between them and you, but they can still be trusted to respect your wishes. Telling them now was your DH's idea, as he wants to tell everybody face to face as well as manage the potential upset they might cause at being told after your first set of parents.
MIL: Will be told in just a week and a half, believes it doesn't matter when people find out your news, but can't be trusted to respect your wishes and wait until you are ready to do it yourselves. Your DH wishes she could be told earlier, but agrees that she can't because of her past behaviour.
You can't treat all these people the same, because they are not the same. They won't react or behave in the same way, so for the minimum stress on you, you and your DH have agreed a plan between you to manage the situation in the best way for you both.
And as you don't see one set of parents all that often, I imagine the logistics of getting them all together at the same time so they all find out at the exact same moment would not be easy.
I said before, to me it sounds like your husband wishes things were different with his mother, but fully understands and accepts why they are not.
I imagine you wish your second set of parents would behave differently too, but you understand that they won't.
Having a difficult family member, or gossipy family member, or one that's quick to take offence is hard work. I learned to my cost that trying to keep a difficult person happy at the expense of your own feelings is not possible.
So I think if you and your DH are in agreement, even if you both wish things could be different, sticking to your plan is the best idea.