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?
OK, I better see where you are coming from now, and it seems more reasonable to me. But, I still believe it can be fairly taken either way on this point. I know she was still betrothed at the time, and my understanding is that process could take up to a year or more to complete. So, we cannot assume that she was getting married the next day. In any event, my reasoning was that the reason she asked the question was that she was still betrothed, AND that she correctly interpreted that the angel meant she would become pregnant before any union with Joseph. (That's what I meant by "right away", I agree she had a normal gestational period.) In that scenario, her question was logical.
I can understand an angel appearing to an old woman and telling her she's going to conceive. That's real news. But in a case like this, it just seems to me that if an angel really appeared to me at this moment, one thing I would wonder is "why appear now?". Under the stress of the situation, I would assume that whatever the angel told me was going to happen, would happen sometime soon, not in some ambiguous future. Of course I can't prove this is what Mary thought, I just tried to put myself in her sandals and figure what she may have thought. :)
Yes; your view would be particularly apt if the angel gave any indication of time, e.g. "Behold thou shalt conceive in thy womb this evening".
There is this historical tidbit though: betrothal was lengthy but it often involved sharing the household with the future wife. The wedding would be a huge feast that would involve many relatives, some coming from far away, and it would consecrate the betrothal, while at the same time requiring the household economy to be already in place at least in the kitchen, if not in the bedroom. Pregnancies would on occasion result form such betrothals; they were frowned upon and tended to accelerate the date of the wedding, but they were not unheard of. This is consistent with the Gospel of Matthew that calls Joseph Mary's husband and indicates that he was thinking of "putting her away" (Mt 1:19), that is, send her out of his house. Thus even if Mary took the angel to mean "you will conceive soon", it would still be a possibilty given that she shared the house with Joseph.