I don't see how not having sexual relations with your husband after giving birth stands as proof that you didn't have sexual relations prior to giving birth. Was Joseph said to be "blessed above all men"? Did Joseph experience this same "making sex superfluous" intimacy with God that Mary did? The only "proof" that Mary could offer of the virgin birth, beyond her own testimony, was the testimony of her husband Joseph's non-involvement in the matter nine months prior to the birth.
What "incontrovertible evidence" could you possibly mean? How does one do that in first century Jerusalem, practically speaking? Did she parade around without pants for the next thirty-three years? Did she make regular visits to her OB-GYN, and nail the up-to-date examination results to the Temple's Western Wall so all could see? How does your "incontrovertible evidence" not have Mary willfully and repeatedly violating Leviticus 18:6-14 for any and all seekers?
Is a partnership with intent to never consummate the union a legal marriage?
Would a Rabbi perform this ceremony knowing the details?
Would this not be mocking God and marriage He created?
Do priests marry many people who intend to be celibate?
Hm. You don't? The Church teaches that Christ was born without violating His mother's hymen. It was, you know, a miraculous birth.
It is very likely that Mary aloowed herself to be examined, and it would not diminish her dignity one bit.
Regarding spousal obligations, by giving birth to the Savior she fulfuilled the essence of it, and of course, the Jewish law says nothing against volunatry abstinence by both spouses. That is even letting alone the fact that this was altogether exceptional marriage.
Protoevangelium of James (Link) is a book written most likely in 2c (not by St. James), and that relates the story that Mary was, acocrding to the Jewish custom, a temple virgin married off to Joseph so that he may provide for her as she reached puberty. This explains her turn of the phrase "I know not man", otherwise without an explanation for a woman about to be married and make children.