"If you put up a Christmas tree and decorate Easter eggs, you're mixing Christianity with paganism." True story, but there is a huge difference between doing something like this and teaching/asserting that Mary was sinless.
"We know that Mary was sinless because the angel Gabriel declared her so ("full of grace")." To declare someone "full of grace" is not synonymous with being sinless. Stephen (in Acts) was a man full of grace also - I assume you believe he is sinless also? And by the way, where exactly does it say that Mary was "full of grace" in one of the 27 books of the New Testament?
Luke 1:28. Actually it doesn't say she "was" full of grace; the angel directly addreses her as "kecharitomene", already-completely-graced-one, as though that were her name.
That isn't how Stephen is described in Acts 6:8. It says he is "plureon cariton" (something like that; the font's not right for Greek in my browser); filled with grace.