Is the Hajj considered valid or void, and what is its legal ruling concerning the omission of Sa'i after Tawaf al-Ifadah in Hajj al-Tamattu', and is it considered the first Hajj of Islam?

Sa'i (the ritual circumambulation between Safa and Marwa) is one of the pillars of Hajj; Hajj is not complete without it, and neglecting it is not compensated for by a sacrifice. Whoever omits it due to ignorance or forgetfulness must return to Mecca to perform it. If one engages in marital intimacy before performing it, a sacrifice (dam) is obligatory upon him, to be slaughtered in Mecca and distributed among its poor, because he has not completed the full disengagement from the state of ihram.