Is it the case that when there is a contradiction between verses of the Qur’an, we refer to the Sunnah to resolve the contradiction, as in the contradiction between the two verses in Surat Al-Muzzammil and Surat Al-A'raf, where we adopt the Hadith: "Whoever has an Imam, then the Imam's recitation is a recitation for him"?

There is no real contradiction between the verses of the Holy Quran, because the Quran is revealed by Allah Almighty, and what appears as a contradiction is only in the minds of some people. Scholars have explained how to reconcile these verses. The Sunnah is not resorted to in order to resolve contradictions in the Quran; rather, it is an explanation of the Quran, interpreting its ambiguities and detailing its generalities. The two verses mentioned in the question do not contradict each other; the verse from Surah Al-Muzammil speaks about the amount of recitation in the night prayer, and the verse from Surah Al-A'raf speaks about the obligation of listening attentively when the Quran is recited.