"Could she have said "no"?"
Of course, just as Lucifer could have said Serviam rather than Non Serviam. Why do you ask?
If she "could" have said no, then our salvation hinged on the whims of a young girl, not on a devine plan from our God.
God's plan, in the very beginning, was for this to happen exactly as it did, with no input, no permission, no discussion from a young girl. God foretold what was to be. There could be no other way for it to happen. It was prophesy, not wild guesses.