Why does the pope have authority over ecumenical councils?
Logically the Pope has authority over ecumenical councils because he is the only member church that is infallible. When the church makes official declarations at the end of councils, the Pope has the authority to speak infallibly.
In normal times, when according to the Divine constitution of the Church, the pope rules in the fullness of his power, the function of councils is to support and strengthen his rule on occasions of extraordinary difficulties arising from heresies, schisms, relaxed discipline, or external foes. General councils have no part in the ordinary normal government of the Church.
If an ecumenical was called today, the topics discussed would be birth control and homosexuality. There is no doubt that the Church would reaffirm its position that birth control is not acceptable within marriage and that homosexual acts are sinful.