With regard to the cooler of food being there, but the LW asking for something from MIL, I can come up with a scenario for that. When I went with my parents on long road trips as a child, my mom would always pack a cooler full of food/drinks for the ride. It was usually stuff like chips, licorice, string cheese, and bottled water; stuff that you could eat in the car without incurring a huge mess. So I could absolutely see the LW asking for something more substantial (like a sandwich) once they'd reached the MIL's house, because you can fill up on chips and snacks, but it's usually not as satisfying. Plus, the LW might have been relying on some of that cooler food being available for the drive back home!
I realize it doesn't cover all the gaps, but I just wanted to throw that out there as an idea.