How can the Prophet’s action of reciting Surat Al-A’la and Surat Al-Ghashiyah in Friday prayer (which makes the first one longer) be reconciled with Ibn Mas’ud’s statement that he (the Prophet) recited Surat An-Najm and Surat Ar-Rahman (which makes the second one longer)?

There is no contradiction between prolonging the recitation in the first rak'ah and what was mentioned in the question, for prolonging the first is often preferred. However, the Prophet, peace and blessings be upon him, sometimes recited two chapters of equal length or the second longer than the first, which indicates that the matter is broad. Some scholars have recommended that the two chapters be approximately equal in length.