What is the criterion for a barrier that prevents water from reaching [the skin], and must it be from outside the body? And does the drying of a small spot on the hand during Wudu affect the condition of immediate succession (muwalat)?

Doubt after completing an act of worship does not affect its validity. The remedy for waswas (obsessive whispering) is to ignore it and not pay attention to it, for the religion is easy, and Allah does not burden a soul beyond its capacity. Moreover, excessive burdening oneself and exaggeration in purification are blameworthy in the Sharia. It suffices to have a strong presumption that water has reached the skin. The drying of a limb during ablution does not affect its validity due to the existence of muwalat (continuous succession) and immediacy.