Is he who sprinkles water and salt in the corners of the house to expel devils sinful, and is his action considered polytheism, despite his ignorance and good intention?

There is no legal, rational, or empirical evidence to prove that salt is beneficial in expelling envy and devils. It is not permissible to consider anything a means of attracting benefit or repelling harm unless it is proven by sacred texts or by sensory experience. It has been mentioned that hanging some grains with salt on a child is an innovation (bid'ah). One who does this out of ignorance or imitation is excused, but it is better for them to avoid it in the future.