The “perpetual virginity” of Mary is not “the historical truth”; it is “the historical belief” of your church. It cannot be proved by logic, evidence or science. There are no means to prove that “perpetual virginity” corresponds to the actual state of Mary and in fact, the contrary evidence from scripture and an extra-biblical source is more probative than the isegesis and tradition of your church.
That being the case your second statement “our Church is a truth-telling institution” fails since the “perpetual virginity” of Mary is a major tenet of your church.
ouch....
Sure it can be. There were people around her who knew her life well. They provided the witness. That witness lives to this day. It is a fact. You may disbelieve it, but it does not make it any less a historical fact.