Some interesting questions which I'm afraid I can't answer with any authority.
Other than to suggest that having seen mature specimens in the countryside, covering old dead trees, the growth appears to naturally be brought downwards by gravity. Without support, the segments can grow outward only so far before they turn earthwards.
I would also guess that allows a better prospects of survival, if the fruit fall near the base of the parent plant, they will at least have that to climb. I've often seen sections of plants anchored higher up having no ground contact and surviving only with those aerial roots having found contact with decaying bark etc.
So I guess the question of how much nutrients those aerial roots take simply "depends" on the growth circumstances as I believe they are formed to facilitate growth first rather than for feeding. However a foliar spray would seem appropriate.
As to forcing, I don;t know.
The growth style seems to facilitate (a) ease of maintenance and fertilizing (b) ease of collection - and maybe they hand pollinate, who knows and (c) allows good all-round sun exposure.
I found this quite interesting video - only partly in English but the language isn't really necessary to see how an established plantation grows them and maybe will help you some.

Here is a link that might be useful: growing dragon fruit commercially