Mothers-in-law are often challenging, especially as houseguests. Try to remember her visit is time limited and therefore her impact on the children will be minimal. The fact that you keep healthy food around the other 51 weeks of the year will far outweigh the one week that grandmother is feeding them low-quality food.
I agree that baby talk is annoying. Can you or your husband discuss your child's speech issues and ask her to refrain from using baby talk? If not, again one week will do no harm in the long run. I hope your husband can understand that you are not being mean but simply trying to aid your children's development. It's nice that your husband sticks up for his mother so you must speak to him in a kind and clear way to avoid turning her visit into a conflict between the two of you.
Dog poop sounds like it's really crossing a line. Choose your battles, but this is the one I would elect to fight. Tell your mother-in-law that the dog needs to stay outside or in a crate if he isn't housebroken or stay home the next time she visits.
If things are really contentious during her stay, it can be a good idea for you to take a few days vacation when your mother in law comes to visit or at a minimum, go out with girlfriends, to a movie or other event that will lower your stress level.
Try to make the most of her visit. Ask her to babysit so you can get some alone time with your husband. And remember this too shall pass and you soon will return to your usual family dynamics.