AFAIK, the payout is based on a weighting assigned by PRS based on the number of public plays. This includes in clubs. Clubs should submit tracklists. It may seem pointless in individual instances, but if everyone did it it would quickly become more meaningful.
Yes. Years back I did a remix for someone and they played it in a mix on Annie Nightingale's show on Radio 1.
It's a weird thing, hearing your stuff coming out of a radio.