First, LOVE THE COOKIES

and the cake idea.
Second, welcome to my boat

I'm in a similar situation as far as the guest list. When we first got engaged, I made it CLEAR that the wedding was going to be small and that the guest list was immediate family only. Period. FMIL has not said a word as to "oh but it would be nice to have..." She has accepted that from day one...and even if she hasn't neither myself of DF has heard anything about it. FMIL is good like that...She's a go with the flow kind of person.
Now, MY mother on the other hand...Not so much. The guilt is just a PITA to say the least...For example, "It would be so nice to have everyone there. Your father would put more money toward the wedding if his family was there." When I got my dress it was "I wish everyone could see you in this dress." "This person is hurt that they aren't going to be there." I could go on and on...but I'll stop there.
Here is how I fixed it...at least for now as she hasn't said anything since this conversation-which I think is in my PJ...
It went something like this...
DF and I looked into a Wishes wedding to accommodate everyone and the cost is just too much. Then, I broke down the cost and the minimum that we HAD to spend at WDW and travel AND dress, shoes, etc. The bottom line $$$ is what ended that dispute. (So far). I also pointed out what she was doing with the guest list. She was making it top heavy with dad's family and cutting out DF's side of the family. Df's family considers "family" to be all of it..Not just adults, but adults and kids. I had to make my mother understand that there was a different definition of family there and she was not respecting it just because she wanted to have everyone there.
Even though they are offering to pay for an extra guest-Sunset Point only allows so many people. Grand 1 only allows so many people-and to upgrade to a dessert party would cost you how much more??? I'd explain that and that you don't think it's fair to try to balance out the guest list for one side when you can't do it for the other.
HTH's
PS mmmmm....Cookies....