What is the ruling on one who performed Tawaf Al-Ifadah but did not perform Sa'i until after sunset on the last day of Tashreeq, and what is the ruling on his Sa'i after this time?

Your Sa'i after the Days of Tashreeq is valid, and there is no harm in delaying it, because it is not a condition for the validity of Sa'i that it be immediately consecutive to the Tawaf. However, it is more complete for it to be immediately consecutive to it, following the example of the Prophet (peace and blessings of Allah be upon him).