Do some schools of thought consider the Sunnah to be overriding and dominant over the Qur’an, and how can this be reconciled with the disagreement among Hadith scholars regarding the disparagement and authentication of narrators?

The phrase "The Sunnah is a judge over the Qur'an" was narrated by some scholars, and their intent was that the Sunnah clarifies and interprets the Qur'an. Imam Ahmad ibn Hanbal hesitated to use this phrase, preferring to say that "The Sunnah interprets and clarifies the Book." There is no harm in the اختلاف (disagreement) among hadith scholars regarding the authentication or weakening of a hadith, for every mujtahid (one who strives to deduce legal rulings) will be rewarded.