I would just stop talking about it and say "You're welcome" later.
Seriously, if your DH is worried about MIL stamina he will be thanking his lucky stars that you have planned a respite every day. I remember how you thought the ADR's out in order to accomodate your DMIL, and IIRC< these are the choices that made the most sense given your daily plans. I would leave them be, and as others suggested, make changes in th event you need to once you arrive.
I had a conversation with a self proclaimed "Diseny Vet" the other day, and she made my head spin! I have no idea what WDW she is familiar with, but it is not the same one I frequent, but she absolutely knew what she knew. People really do not understand the benefit of a meal set in place unless they are looking for one, and cannot find dinner on the fly.
If you were on a CS plan, and your DMIL was onboard, with canceling, I woudl consider it. But you have a prepaid plan that will only be a value if you use it, and the odds of you using it without plans are slim. AS one poster suggested, you will be able to enjoy all of Nine Dragons menu items. LOL!
You did well, leave it be for now and see what you need to change when you all are there and have at least tried it your way.