Well, MY grandparents don't watch the girls. MY grandparents are in their mid-to-upper 80's. My Gram falls and hurts herself at the drop of a hat and can't drive any more. My Grampa has no patience for children after half an hour.
As for my kids' grandparents, I don't impose rules on them any more since they're out of the toddler years except for certain food rules regarding food allergies and intolerances and trying to avoid certain behavioral triggers for my autistic daughter. I'll leave ADVICE for them rather than impose rules.
The only steadfast rule is that I'm able to get in touch with them in emergencies and vice verse. Otherwise, their house, their rules. The girls are expected to be on their best behavior.
ETA: I forgot to address something. The fact that they were so late in bringing the kids home without contacting you or being available for contact is a huge problem to me. They HAVE a cell phone but leave it off or won't pick it up? Then they need to check the cell frequently for messages. What if something had happened to your or your husband, let alone the kids, and you had to let THEM know rather than the other way around? Had your parents tried contacting YOU and not been able to get in touch with you, would they be upset with YOU?
As for the meal time prep, if your mom wants to prepare multiple "main" dishes, that's on her. She can cook whatever she wants in her own home for her guests. It's not setting a precedent for your children in YOUR home... and if your mom chooses not to do it in the future then your children will cope well because of how you handle it in your home. You're a great mom.
As for naps, well... yes, you'll get some fallout at home from how they handle that but just think... your parents are getting fallout too. ] :-)