I agree with Labug. I spent most of my M life upset at my in-laws (mostly MIL). Being M to a "mama's boy" isn't fun. Some things you can't change. As hard as you try...you'll never change it. One day I woke up and realized that I didn't have to go to my MIL's five nights out of the week and all day on Sunday. If my H wanted to go, fine. If he wanted to take the kids, fine. If she wanted to come over, fine. But I was an adult who could make my own decisions.....and was prepared to realize some folks would not like my decision. I can't tell you how free I felt! Of course, I was still pretty young then, but it relieved me from a lot of stress I would have every time I went to my MIL's.
I know what it is to have resentment toward your H that was influenced by your MIL. I also know what it is like to be M to a passive man. I don't know that I could have as big if a heart as you, and try to set up time for the kids to be with her if she didn't put forth more effort than is apparent. As a mother, you don't want your kids missing out on the R they can have with their paternal GM. It makes sense.
If there is anything I can pass along to you, it's the fact that as we grow older we discover just how important peace is to our lives. I'm at the place that it's really better for me to avoid as much stress as possible. Peace is the center of our well being. So, what I'm saying here is....if this causes you to feel uncomfortable, fake, or whatever....then you don't have to do it anymore. At the least....you don't have to do it before the next time school is out. If you feel ready by then, fine. If you're not ready....fine.
We can't make all the relationships work in our kids' lives. You can't be their dad. You can't make their GM want to spend time with them. (BTW, she kind of sounded like she was doing a favor, but IDK.) As much as we want our children to have enriched lives from those related to them, and those who have daily contact with them......we just can't control it. Ha...another thing I had to learn was stop trying to control what I couldn't. Not that I think it's what you're doing. I think you're just trying to be a good mom.
So anyway, I said a lot to not say much!
Just give yourself some breathing room for awhile. It takes a lot of inhaling & exhaling to get resentment out.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!