When I looked at the 100% Co Op category I was really confused, because in the Co Op menu there's still all 55 recipes that you can choose, and you only do the first 10. Why would a category be called "100% Co Op" when its not even close to 100%?

It’s because when you load up a new file you only get the 10 recipes and can’t unlock any unless you play 1P mode. I was following the rules at the time when I first played it and that’s how I thought it worked. Wouldn’t make sense to play both 1P and 2P in the same run. (It said timing starts on new file select)