Is fasting on Ekadashi and breaking fast on Dwadashi an ordained kainkaryam and done out of one’s own desire? When those who have undergone prapatti observe this, will that be an obstacle to attaining moksha?

Ekadashi and Dwadashi are ordained kainkaryam and are mandatory observances. One must fast. Failing to do so will accrue sin. But this sin is not serious enough to hinder moksha. But one will be forced to pay for the sin in this very life.