Hi all. My name's Scott, I'm from the UK, and I've just registered with Websleuths solely to share with you my thoughts on this case.
I wonder if anybody else's noticed that although Dorothy claimed the purpose of her trip was to buy a dress for her sister's party there are apparently no reports of anybody seeing her in any of the fashion stores that, then as now, must have thronged Fifth Ave.? It seems to me inconceivable that a prominent Manhattan socialite such as Dorothy should have been able to roam these shops completely unrecognised, searching against the clock for that elusive party "number". One might well ask; did she actually go into any of these shops on that fateful day? And if she in fact didn't, why might that have been?
It's here that a couple of things from the American Heritage article strike me particularly. Assuming all its details are true, 11 or 12 weeks would already have elapsed since her dalliance with Mr Griscom in Boston on the 16-24 Sept. This is almost exactly the point at which an unborn baby starts making itself visible in the course of a typical pregnancy. Could this be why Dorothy dissuaded her mother from joining her on her trip? Some quick research on Google suggests a tight-fitting dress (particularly at the waist) was still the fashion for young ladies in 1910, which would obviously have presented some difficulty to a pregnant Dorothy. Also, she would in all probability have required another woman's help in the changing-room and thus be at risk at giving her secret away. Even if this had been a shop assistant rather than her mother or a close friend, she (Dorothy) surely was not unknown enough that tongues would not start wagging around the town.
I imagine it would almost have been out of the question for Dorothy to choose something from her own wardrobe instead that would still fit her comfortably; this was after all her own sister's coming-out party, and her friends would not fail to notice if she was wearing "some old thing" and, accordingly, move her a few rungs down the social ranking for showing her sister up. It's hard not to think that such thoughts must have been playing on her mind endlessly as she attempted to keep a brave face on things, whilst time was steadily slipping away; it must almost have felt like the final straw when that friend of hers handed Dorothy her note of acceptance to the party during their chance encounter at the bookshop.
What do I think happened after that "final" sighting, given the foregoing? If the American Heritage article is correct in all particulars, then I can think of at least two possibilities; a) that Dorothy did return home, perhaps with a real or feigned headache (hence her mother's enigmatic telephone reply), and at some point blurted out her secret, possibly in answer to why she still hadn't found a dress etc. when the party was only five days away, or b) she fled to a friend's and only got in touch with her family some time afterwards as has already been suggested in this thread. What next after that? I think it very unlikely (though of course I can't discount it totally) that Dorothy died on the abortionist's table: such news would have destroyed her mother utterly, yet she lived on for another 18 years. It could well be she refused to undergo the procedure and so her father saw no option but to banish her from the family home permanently (to Europe? Recall that he was a perfume importer and so would have been in a position to quickly spirit Dorothy away to a new life overseas with the help of some trusted business contacts) and maintain the "missing daughter search" pretence to keep scandal at bay.
I've found a weblink
http://coloradoclues.com/Vital%20Statistics%20S-U.htm which, if the info in it is to be believed - see the entry under "Seeley, Dorothy" - suggests another, perhaps additional, reason why her family might have sent Dorothy away; i.e. she refused to go through a shotgun marriage. Something which I have to say seems to be very much in keeping with Dorothy, from what little we know of her.
That's all I have for now. A happy Christmas to you all!