You are well and truly being taken advantage of op.
I get your DH asked you once and then twice and it's all the frog in the pan situation. That though whenever anyone has collected my Dd for me and had her if I have been unavailable I have been grateful and mindful of not taking advantage.
Since there wasn't a conversation I wonder if DH slipped into the thought process of - ah actually it would be easier to ask you. Rather than him leave work or ask mum (who would kick off). You by not saying hey this impacts me became the easiest option by default. If he had had a chat with you, then it risks you potentially saying actually no.. I don't wanna sign up for that.
I can see you have had the usual comments 🙄 thing is just because you married your DH doesn't mean morally or otherwise you have agreed to be unpaid unappreciated childcare that can't quit. Even childminders can say no and they get paid. Sure others will say they do that and that's fine but ultimately that's their choice and what works for their family.
Make you being pressured into this, the harder option for DH to get out of this bind.
It can't be your default unpaid childcare and also you have no say in xyz and your not even given the respect of being called by your own name.
I would say no and if DH doesn't or won't make other arrangements, show him the door to your house and shut it behind him.
That will force the parents to parent without you. And I suspect that won't be as appealing as the current set up.
Ps you don't have to justify how long you need to get ready to randos on the internet. You are not property- and neither are you owned by DH or mum.
My Dd has a sm and I would never treat sm like this ever.