Actually, I can’t get too far into this, but the tradition that Mary disappeared from her tomb and was assumed into Heaven goes back to very early days. In the very early middle ages, some of the Irish and British missionaries went to the Holy Land to visit the place of her tomb.
Jesus preceded Mary, of course, because according to the bible he died and ascended into heaven well before she did.
The Hebrew Bible tells us that Elijah was assumed bodily into Heaven, and when Jesus meets with him and Moses on the mountain, the presumption is that both of them were.
They were saved by the grace of Jesus extended back in time, according to traditional interpretation.
Mary was conceived without sin, as a suitable Mother to the Christ, and therefore did not suffer from the effects of Original Sin, which brought death into the world. That was the belief of early Protestants as well as Catholics. Luther and Milton, to mention two.
The purest of all God’s creation in Heaven and on Earth, must surely be assumed into eternity to be with her son and the Son of God incorrupt from bodily decay. She alone was conceived without Original Sin. To argue differently, wold be deny the basic predicate.
This vision was made in connection with the announcement of God's approval of His Son as a faith building event for these disciples.
As for Elijah the use of the term heaven or heavens must be understood in the light of what Paul said seeing that Elijah was most certainly flesh and blood and the fact that Christ had not yet offered his sacrifice on the altar in heaven.