Mary was the "mother of God" in the physical sense in that her Son, Jesus Christ, was God in the flesh.
Obviously she's not part of the godhead or trinity, but I do think she has an exalted role. My old Protestant pastor said to me once about Marian apparitions (which the church takes some trouble to check) that "There's been a lot of them" and "I think they're a judgement on a divided church".
If we venerate Mary, she in turn points us to her Son and implies "Do whatever He tells you". I think Protestants undervalue her importance, and the saints for that matter. To quote my old pastor again, "They're (saints) doing something up there!"
If an archangel turns up and tells you you're blessed, you've got a role to play, and it won't stop with death. When Christ spoke to John and said "Behold, your mother!" and to Mary "Behold, your son!', he was standing in as proxy for the whole lot of us.
Getting back to the OP's original point about the transfiguration, I take it in the literal sense - that Moses and Elijah turned up and conversed with Christ while Peter, James and John looked on, while nobody else could see what was going on as they were hidden by a "cloud". At the time Christ went so far as to tell them not to tell anyone what they had seen until He had been raised from the dead.
For the same reason I don't have any trouble in believing in Mary's assumption to heaven - Elijah was seen riding a fiery chariot and Moses's body couldn't be found. I think he was assumed, which was why they could not find his remains. In due course he and Elijah were at the transfiguration representing the law and the prophets.
I think the three main figures of Jewish theocratic history were all assumed into heaven, representing the law, the prophets and the birth of Christ, ushering in the new covenant.
It was a "vision" in the sense that the three core disciples of Christ's team could literally see it.