And who is the witness par excellence? In every meaning of the term.
John the Baptist, maybe Saint Paul. Who knows. But it wasn't Mary. You know why?
Yes, because of Rev. 3.14: Καὶ τῷ ἀγγέλῳ τῆς ἐν Λαοδικείᾳ ἐκκλησίας γράψον· τάδε λέγει ὁ ἀμήν, ὁ μάρτυς ὁ πιστὸς καὶ ἀληθινός, ἡ ἀρχὴ τῆς κτίσεως τοῦ Θεοῦ·
The "witness" par excellence is Christ, the "apostle" par excellence is Christ, the "prophet" par excellence is Christ, the "evangelist" par excellence is Christ, the "confessor" par excellence is Christ, the "virgin" par excellence is Christ, etc. But we don't typically refer to Christ as any of these things as a common practice, even though he is all of those things. It's all over Scripture if one has eyes to see and ears to hear.
Our Lord fulfills all the requirements of such categories, but also exceeds them. If he didn't do the former, none of those categories would have the meaning they have in our tradition; if he didn't do the latter, he'd just be an over-achieving Boy Scout collecting merit badges. All of those "titles" and "categories" are rooted in him, they derive their meaning from him. It may not be common to call Christ a martyr, but if he's not, then martyrdom is meaningless.
In a similar way, but in a way lesser than Christ, Our Lady "is" a lot of things we don't normally refer to her as. Who brought the Word to all nations more than she did? She didn't travel as much as the apostles did, but the Word they preached entered the world as the Son she brought forth: she is certainly "apostle" and "evangelist", even if not by the standard definition, because she fulfills its more fundamental meaning. She didn't write any books of oracles she received from God, but her whole life was a proclamation of the Word of God which came to her, first through her obedience and then through her consent to the Angel's message, after which she proclaimed to Elizabeth and to all humanity the greatness of God her Savior: how is she not a "prophet"? If we are limited in considering martyrdom as involving the shedding of one's blood as a witness to God, then Mary, having died peacefully at a good age, may not "count". But if we consider the fundamental meaning of martyrdom, of witness, then she is certainly that.
Did she not witness faithfulness to God in the face of darkness and death more than all the other martyrs? Our liturgical texts (here, I specifically refer to Syriac texts, though I'm sure the same sentiments can be found in Greek and other texts) speak of the martyrs having seen Christ in the heavens holding crowns plaited by the Holy Spirit, waiting to crown them upon their death, and their response was "Let us die for him who died for us". Even St Stephen gives up his life, after believing in the death and resurrection of Christ, and seeing him sitting at the right hand of God. But Our Lady answers the Angel's lofty words in Luke 1 with obedience, and from then on, she's subject to people's doubts about her virtue, she flees to Egypt to save her Son, and when he enters into his ministry, you know the ridicule that fell on him was directed toward her (that's just how people are). When she sees him die on the Cross, does anyone think she was there smiling and winking at the bystanders saying "We'll see who has the last laugh on Sunday"? No. We believe she had an invincible faith, in spite of the fact that nothing the Angel told her seemed to have come to pass: when he's taken down from the cross, no one thinks he's great or the Son of the Most High, he can't sit on the throne of his father David if he's dead, and so it seems like his kingdom has ended before it had a chance to begin. Is it so easy to believe in the resurrection for us, even though we have the benefit of two thousand years' space and innumerable witnesses since it happened? How much more difficult must it have been three days before it happened, how much it must've looked for all intents and purposes like all was lost and meaningless. And yet, she had faith even when the sword prophesied by the Elder Simeon in Luke 2 had in fact pierced her own soul: not a physical martyrdom, but a martyrdom of the spirit, the very martyrdom that must necessarily precede one's willingness to suffer physically and shed blood unto death.
Like her Son and because of her Son, Our Lady fulfills and exceeds all our categories. She didn't shed her blood as a martyr, but if she's not a martyr, then none of the rest are either. As long as we're stuck on the physical violence and blood of martyrdom, we'll not only misunderstand the "martyrdom" of Mary, but we already demonstrate a misunderstanding of the martyrdom of the martyrs we do accept. So, with all due respect, I believe you're missing something in your analysis: the most important things are meant to be understood in the heart and not just in the mind, and this is one of them.