“I once read that the teaching about the Assumption of Mary came from St. Thomas (you know, “that” Thomas...*doubting* Thomas).”
Not that I have ever heard, but that doesn’t mean that he didn’t preach about it. I did hear that he was in Jerusalem when she died and angels transported him to Ephesus to be at her funeral.
Here’s an icon of the Dormition. Notice Christ holding his mother’s soul:
http://acrod.org/assets/images/Iconimages/Dormition1.jpg
Doubting Thomas, the Apostle, or Thomas Aquinas?