Start with no assumptions. Just read each individual text on its own. Is Mary included, by name, at the cross, in Matthew, Mark, or Luke? The answer is no. I am not saying that she wasn't there...however, it is reasonable for someone to note she has a large "cult" following despite no mention of her in 3 of the 4 passion scenes. And I would guess Geza Vermes gets into greater comparative detail about her roll (vs the silence on her roll) in the differing accounts.
Other groups that believe Jesus was the messiah, but not part of a trinity, here:
http://www.biblicalunitarian.com/modules.php?name=News&file=article&sid=113
http://www.torahofmessiah.com/
http://www.sabbatarian.com/
http://www.tcpc.org/about/the_8_points_english.html
RE: "The changes he made to Protestant theology to justify founding a new church went against over 1600 years of interpretation by his direct fathers in faith."
That's pretty much what the Jews say about Christians who read their own version of truth into ancient Jewish scripture, totally negating the original meaning.
RE: "Protestantism is a small minority of Christianity worldwide"
The number of people in a group does not determine whether or not something is true.
RE:"She is indeed mentioned after the Resurrection, in particular at Pentecost."
Sorry, i meant "at" the resurrection as discussed in 31 and 33....with this link to Vatican commentary:
http://www.catholic-pages.com/bvm/resurrection.asp
All the blessed verses w/regards to Mary are in Luke as you point out. Look again at the last one you quoted (quoting Jesus) . Here's the KJV:
"27) And it came to pass, as he spake these things, a certain woman of the company lifted up her voice, and said unto him, Blessed [is] the womb that bare thee, and the paps which thou hast sucked. 28)But he said, YEA, RATHER BLESSED [ARE] THEY THAT HEAR THE WORD OF GOD AND KEEP IT.
and the NLT version: 27) As he was speaking, a woman in the crowd called out, "God bless your mother-the womb from which you came, and the breasts that nursed you!"
28) He replied, "But EVEN MORE BLESSED are all who hear the word of God and put it into practice."
I went over Luke 1 26-56.
Instead of saying she was "unaware" in Luke 2: 39 -50, i should have said it looks like she "forgot".
RE:"It's recorded by two Gospel narratives and is an explicit sign of the fulfillment of the Messianic prophesy."
If you are referring to Isaiah 7:14...that would be a perfect example of reading another version of truth into the ancient scripture, and totally negating the the original meaning. This article discusses it in context of Dead Sea Scroll discoveries...and the Vatican take on it: http://www.lightwatcher.com/spirit_stars/bible_scrolls.html
RE:" But if you won't believe in God, then this would naturally be difficult to accept."
You assume that if one does not accept the Virgin birth story then he/she must not believe in God? Many people believe in God, just not in the same way you do.