Weevils eat through food and compromise the nutritional quality. They lay eggs inside grains and then the hatchling weevils hollow out the grain until they finally emerge and eat through the rest of the food. Freezing should kill the hatched weevils but possibly not the eggs, and it won't undo any damage the weevils have already done.
This is a tub of food I was recently given which was infested (it went straight in the outside bin after I took the photo, and the tub was boiled). You can see they've eaten holes through a lot of the pieces. I think the dust you can see at the top is essentially what the weevils excrete after eating the food. You can't see any weevils because they all went into hiding when I opened the tub (they hide inside hollowed out grain husks).
I've heard so many reports of weevil infestations in pet food this year. I think it's either the weather or the manufacturers have a storage problem.
If she's eating more veg than usual, that could be the cause of the diarrhoea. I would cut out the veg for now and just feed dry seeds and cereals such as oats until you can get more food. If the diarrhoea continues after stopping the veg and especially after she's back on her normal diet, she should see a vet though.
I don't gradually introduce new foods and it's always been fine. I don't even always feed the same food from one day to another. The idea of mixing foods together to gradually introduce is mostly for rabbits and guinea pigs which have much more sensitive digestive systems.