Is the Hajj valid if the Sa'i (ritual circumambulation between Safa and Marwa) was performed after the Farewell Tawaf due to ignorance of the Sharia ruling?

Your Hajj is valid if you fulfilled its pillars and obligations. The additional Sa'y after theTawaf Al-Wada' does not affect the validity of the Hajj due to your ignorance that it is not prescribed. Nor does it prevent the Tawaf Al-Wada' from being the last act of worship performed at the Ka'bah, as 'Aishah (may Allah be pleased with her) did.