What is required of one who intentionally missed the Maghrib prayer, thinking it impermissible to pray behind an Imam who lets his garment hang low (musbil), and was unable to make it up at the time?

Firstly, one must repent to Allah for delaying prayer beyond its prescribed time without a valid excuse, and then make up that prayer. Delaying prayer intentionally is a more severe sin than isbal (letting one's garment hang below the ankles).

Secondly, the prayer of someone who practices isbal is valid for them, though it is disliked (makrooh). The hadith that mentions the non-acceptance of their prayer is interpreted as referring to the loss of reward, not the invalidity of the prayer itself.

Thirdly, praying behind someone who practices isbal is valid, though it is disliked if it is possible to pray behind someone else. The Companions prayed behind individuals who were more outwardly sinful than those who practiced isbal, and the majority of scholars hold that prayer behind a corrupt person (fasiq) or an innovator (mubtadi') is valid.