You are not suggesting that Mary was sinless from birth, right? Because I can see nothing in Scripture to support that. I certainly believe she was forgiven of any sins and thus made "sinless" when the Holy Spirit came upon her. But I do not believe or see any Scripture to support the idea that Mary was sinless or a perpetual virgin after the birth of Christ.
Plus a whole lot of OT prophecy
I wouldn't say this constitutes dispositive, or shall we say "mathematics-syle" proof (which you never get in Biblical foretelling and foreshadowing) but rather converging lines of evidence implying a reasonable conclusion.
What's really dispositive is the Church's authoritative conclusion from the evidence, by the power of the Holy Spirit.
Before you dismiss this last one, consider that this is the same authority which established the basic Christology and the doctrines of the Incarnation and the Trinity --- also derived by reasonable inferences from Scripture and confirmed by the authority of the Church --- which are accepted by all Christians (with the exception of he Mormons.)
that's as good as I can do before eight in the morning. Have a blessed day!