ellisbell, discussion has moved on a lot, but I'll try to respond.
Yes, actually I do keep in touch with some candidates not admitted, though they are obviously a self-selecting sample in that I don't chase them. Some of them - again from all sectors - want to reapply in the following year. Some of them go elsewhere and keep in touch.
Also FWIW, I currently have a brilliant graduate, ex-Durham, who was rejected by another college as an undergraduate. Also FWIW, I have SEVERAL tenured colleagues who were rejected by Oxford as undergraduates. So of course I know that not all clever/able/determined people end up here.
We should also talk about how much harder our students work than those elsewhere, though.
I can also only speak from my own experience, but your account above of how that grey area in the middle plays out at admissions doesn't match my experience at all. Typically we - all the admissions tutors - pick over every aspect of the application all over again. Not all those from either advantaged or disadvantaged backgrounds can get an offer because there are many many more of them than places for them, and this means of course that we often turn down very well-qualified people from all sectors.
It would be stupid to say we never make mistakes - we're human - but I don't think our mistakes follow any particular pattern of the kind you might imply. What I can say is that we sweat blood to admit people on academic merit as we see and find it.
I did not myself go to an independent school and neither did my subject colleagues, and FWIW neither of them has a cut-glass BBC voice.
In top US schools, admissions are conducted by professional administrators and not by academics, leading to a system which overtly favours donors and alums. If you get heard, ellisbell, it will probably go that way here too, and while the dons are not perfect, I think a tickbox system would be a lot more liable to corruption. At my college we've turned down many a millionaire's scion. This is actually made EASIER if the college is rich.