Is there a Sa’i for the pilgrim performing Qiran (combining Hajj and Umrah) after the Ifadah Tawaf if he has already performed Sa’i after the Umrah Tawaf? And why is his first Sa’i not considered sufficient for the Sa’i of Hajj?

The Qarin (one performing Hajj Qiran) only needs to perform one Sa'i for both his Umrah and Hajj, because the rituals of Umrah are subsumed into Hajj. Scholars have explicitly stated that his Sa'i suffices for the obligatory Sa'i, based on authentic Sunnah. So, if he performed Sa'i after the Tawaf al-Qudum (Tawaf of Arrival), he does not need to repeat it. If he did not perform Sa'i then, he is obligated to perform it after the Tawaf al-Ifadah (Tawaf of Departure from Mina).