I think that's a huge stretch. The angel says "You will be with child and give birth to a son ..." Now, think of the setting. An angel of God appears to her out of no where and says she is going to be with child. Why should Mary assume that this angel has come to tell her distant future? This was all brand new to Mary and the scripture even says she was afraid. She thought the angel meant right away, and the angel did mean that. That's why she asked her question.
Prevail means that the Church will lead all faithful souls to Christ as He comes in glory. Scripture is but a tool of the Church in that process. How do you define "prevail" with the scripture?
Well, it wasn't my idea to use the word "prevail" with or without scripture, so that's a hard question to answer. God's Church will prevail with the scripture when Christ returns. I believe that God's Church will be immeasurably larger than it would have been without the scripture. But of course, God could have done it any way He wanted to. The elect are always going to be the elect. I just know that I am much closer to God than I would have been without scripture.
The natural assumption to Mary, if your theory about her were true, is that she will get married and get pregnant and have a baby son, who will have a great future. She was betrothed, remember?
The angel did not mean "right away". He said "thou shalt conceive in thy womb, and shalt bring forth a son",-- indicating normal pregnancy,-- and ideed it took her the usual gestational 9 months, as the scripture tells us. Nothing in his speech prior to Mary's question in v.34 indicates anything extraordinary regarding the physiology of Jesus' birth. Where has your school of plain reading disappeared?