When the GK's were smaller and had to feed them I just bought jars and fed them that. DD started getting them for me so I then just bought some when I did the shopping.

Maybe this is the way to go, send enough food for the time she is with your MIL and say you are trying these foods in case you need them when out.

As for anything acidic, definitely put a stop to it. I cannot have anything like pineapple, orange etc. because of the acid content and if she reacts then you have a very upset child.

As a grandmother I say tell her where you stand. She is your child so grandparents should respect you in the way you bring up your child/ren. Sure, things are different to when we had small children but in alot of cases the changes are for the better as we are better informed.

All the best as you deal with these issues.