Sorry for such late comments; Tuesday and Thursday evenings are the only time I get to concentrate. And this one took up all of tuesday.
Re. 4496, think I was lucky to realise that the artificial constraint was going to be important. 10d and 12d are such unusual words that it couldn’t just be coincidence. (There is a very interesting website of linguistic curiosities at jeff560.tripod.com for anyone of a similar bent.)
A magnificent rendition of this self-imposed restriction and, to my mind, all are “proper” words. However, I do feel I am missing the point as to why ——-A was chosen for the endgame.
Am I being dim?
Also spent an absurd amount of time trying to reconcile HOIST, PALE and FLY in the extra letters before realising it was a bit easier.
As they say: a little learning is a dangerous thing. Exceptional construction by MynoT and thanks for a title that gave us a good clue.