Could my mother-in-law's act of putting all the kitchen utensils and flour in the toilet, keeping only the refrigerator and stove in the kitchen, and causing constant problems in the house, be due to the influence of devils and jinn possession?

There is no harm in placing utensils and belongings in a place where they will not become impure. If they do become impure, the impurity must be removed before use. It is preferable to look for another place to avoid impurity. This should not be a reason for Satan to gain dominance. If problems are found, their causes should be examined and treated in accordance with what the Shari'ah dictates, while frequently performing Ruqyah and diligently observing the prescribed supplications.