Must one make up missed prayers due to not knowing that a small amount of discharge from the anus is not excused, especially when suffering from Waswas, and is there a sin for not making them up?

This statement is correct and is the view of the Hanbalis. What exits from the anus is not among the pardoned impurities. The Hanafis' view is to pardon a small amount of grave impurities, and there is no harm in following this view for someone afflicted with waswasah (obsessive whispering). Scholars have differed regarding the ruling of avoiding impurity in prayer: some said it is an absolute condition, some said it is a condition with knowledge and ability, some said it is obligatory, and one who abandons it is sinful but their prayer is not invalidated, and some said it is a non-obligatory sunnah.