my impression of it is that (and I'm just guessing, mind you) this little mousy girl feels so trapped that she really believes she only has the one opportunity to escape--to try to find the strength to reach for a destiny other than her aimless life in that little town. When she fails, the failure seems permanent (as failure often does to young people). She sees no other way out and *wills* herself to die--in a sense, committing suicide. This would certainly square with the intense emotional pain she's in throughout the play.

The dead mother character bothered me for a while until I decided that she was simply trying to save her daughter from her likely (or inevitable?) fate by saying that the man was "hers"--trying to tell Mary (through Dad) to be satisfied with the life she had, as she knew that only catastrophe awaited if Mary tried to go forward with The Man. (How did Mom know that? Well...she *is* dead, after all!)

But yes, allegory, symbolism, definitely. At least I hope so. I dearly love this episode...It would be nice if it made some kind of sense!