I do not think you're being unreasonable, first of all.
I see at least two separate issues here, though. First and most worrisome is that IMO the two of you should always be making plans together and agreeing on them before allowing anyone else to be brought into it. Instead, if I understood correctly, your husband promised you one thing, then changed the plans to include and focus on his mother instead, without your agreement. There was also the issue about the ill child that you apparently weren't consulted about by him first. That's a big problem. Or it would be to me. It should be the two of you always at the center, not him making unilateral decisions or opening the door to include his mother in those discussions.
He is also setting you and his mother up as opponents for his affection. (And careful there. I really think some spouses at some level like being fought over, even though it does cause them so much strife and chaos at the same time).
As far as the second issue, who gets to be special on Mother's Day. Well, first, there's nothing particularly sacred about that calendar date. And as you say, you weren't even planning to celebrate on that date anyway. Regardless, there is plenty of room for both you and his mother to have your special time and no need to compete for it. You don't have to divide up the day, either. One weekend day is hers and the other is yours or whatever.
I would first insist that he agree that from now on, the two of you discuss things and come to a mutually agreeable solution, before his mother or anyone else is included, as a firm standing rule of your marriage. And he also needs to stick by that agreement, not go changing it later, whether on his or with his mother involved. If he refuses, then marriage counseling pronto because that is just not going to work. Fortunately, you don't seem willing to just put your head down and submit to such poor treatment.
Then, as far as Mother's Day, it sounds like his mother is competing with you, instead of acting like the older and wiser person and saying, "You kids have your special day. I'll see you next weekend." And her demanding gifts, ugh. Many of us older people don't like our grown kids to spend their money on us. She doesn't sound particularly evolved. It's unfortunate that she's not more mature and/or doesn't remember what it was like to be a young mother. I think she should be supporting your family as a family elder, not competing and making demands.
But one great way to take the air out of her sails on that game imo is to just not compete back with her. That is not the same as you giving in and her "winning" the competition. It means you graciously insist that she have her Mother's Day celebration on Saturday and then your Mother's Day with your husband and kids is the next Saturday, or whatever. At the root of the competition is the fear of being forgotten or replaced.
When someone is in that tug-of-war mode, generously giving her what she wants might make her stop feeling like the two of you are fighting over your husband/her son's affection. And then, possibly, she'll relax and she'll stop. After all, there's no reason to jealously, defensively, fight to get something that is freely given.
Also, divide up that labor! It's not automatically your job to do everything when you have company. What he doesn't know how to do, he can learn the same as the rest of us had to do. Don't let him get away with acting helpless. He is not a child. Or he can find a way around doing the work himself, like getting takeout food and/or hiring a housecleaner. What would he do if he was single?
If this is a recurring problem, I highly recommend marriage counseling. A few sessions could probably straighten it out. Good luck.