All in all, I am very much in agreement with your interpretation. But you never did answer, is the slang usage of "beat" as tired, common outside America? I'm just curious now, you guys may very well have invented it for all I know.
"She opened strange doors that we'd never close again"
Also of interest is Aldous Huxley's book The Doors Of Perception, which I haven't read, but I think is about his initial experiments with mescaline. Doors of perception, "she opened strange doors." Just a little note regarding your earlier statements of strange doors being an entrance to other states of mind.
"She asked me to stay and I stole her room"
I think "room" is always a key term. I've never counted all of the Bowie songs in which "rooms" are referred to, but there are a lot of them. Rooms are small spaced floating in my tin-can even. I think "room" represents the mental landscape, and more specifically, mental isolation or alienation, whether it be literal social isolation, or non-physical isolation. Cocaine could change his mindstate, change his room. She asked him to stay, and when you ask someone to stay you usually don't mean permantly, but he stole the room because he wanted to stay in it forever, he couldn't handle going back to his old room once he knew there was an escape, but now, he's wanting to abandon this new room too.
"She asked me to stay" could also be a subtle reference to the track Stay on Station to Station, and considering both tracks, this might suggest a connection in subject matter between the two songs.
I am Greta Garbo