I believe grandfather is correct.
The Blessed Mother’s
fiat was a requirement of salvation history. In the Melkite rite, they say, “All creation held its breath, wondering what her answer would be.”
It is however an ultimately meaningless hypothetical question. She had been prepared since the dawn of creation for her role, and all of human history arranged for her place. A people had been created, and set apart from the the pagans, and given Laws that kept them separate. A promise had been made, that the Davidic Line would produce the Messiah. A child had been conceived without sin fifteen years before. The child was educated in a Temple to understand the Davidic Promise. A Herald had been conceived by a miracle, just six months before, “to go before the Lord to prepare His way”.
All of creation had been ordered for this moment, from the instant Adam and Eve had fallen.
She could have said “No”, in the sense that her human mouth could have had said the word.
((How foolish to think such a thing… God asking someone to do his will, that there may be the possibility that they turn God down, this has to take the cookie… God in all his wisdom and foreknowledge…
But all that she was, and had been prepared to be, shouted “
Fiat mihi secundum verbum tuum”.