2. The Incarnation itself is proof of her sinlessness and that is in Sacred Scripture. As to her sinlessness, how do you support the idea (without getting into gnosticism) that a perfect sinless GOD could not be carried and birthed by a sinful mother?
The Holy Spirit (also perfect) can dwell in us and there is none of us that has never sinned. I would submit that it was entirely possible for Mary to be a normal girl that sinned occasionally and still be the mother of Jesus.
That is something that needs to be proved in scripture that the Incarnation required a sinless vessel. Your statement assumes this as fact. I submit that God is bigger than that.
At the incarnation, the angel referred to Mary by a new name, in Greek, kecharitomene. It is used nowhere else in scripture. Here is analysis of this word:
" 'Highly favoured' (kecharitomene). Perfect passive participle of charitoo and means endowed with grace (charis), enriched with grace as in Ephesians 1:6 . . . The Vulgate gratiae plena [full of grace] "is right, if it means 'full of grace which thou hast received'; wrong, if it means 'full of grace which thou hast to bestow' " (A.T. Robertson, Word Pictures in the New Testament, p. 14)
"It is permissible, on Greek grammatical and linguistic grounds, to paraphrase kecharitomene as completely, perfectly, enduringly endowed with grace
." (Blass and DeBrunner, Greek Grammar of the New Testament).
The word, which the angel Gabriel addresses Mary by, stands for being completely, perfectly, enduringly endowed with grace. It is a perfect passive participle and testifies to the fact that Mary has always been, is, and will continue to be endowed with grace. But what is grace? Grace is that power from God to withstand sin. Kecharitomene
is the name that demonstrates the Immaculate Conception, that she was endowed with perfect grace from the beginning of her existence, and would always continue to be endowed with that grace.
There is more support than just this word, for example, the enmity promised between "the woman" and "the serpent" in Gen. 3:15, as well as "the serpent's offspring" and "her offspring" (who is Jesus). And also the fact that Jesus fulfilled the commandment to honor his mother and father perfectly. It was within God's power to do it, it was proper for him to do it, therefore, he did it. Finally there is a tremendous parallel between Mary and the Ark of the Covenant. You probably can understand the implications of such a parallel better than most on this forum!