Does the foot become impure if it falls into bathwater mixed with drops of menstrual blood? And is it sufficient to pour water on sandals to wash them without thoroughly cleaning their insides?

If the water in the basin is pure (tahir), then placing one's foot in it does not cause harm. The fact that the basin sometimes becomes impure (najis) due to menstrual blood does not mean it is always impure. As for purifying the shoe, it is sufficient to wash the impure part of it, whether it is the outer or inner surface. It is advisable to disregard obsessive doubts (waswasah).