Have no idea on the % of the population that has played chess.
The average ELO is probably something like 1000. On the park/bar question if you are talking about just playing random people that happen to know how to play, a 1400-1500 rated player will likely win a vast majority of their games.
I dont think the OPs questions can be answered accurately . Chess players in the bars/parks of the US wont be as strong as those in eastern european countries , for example . I agree with you Martin that , in the US , a C class player will do well against bar/park players ... who are just casual/recreational players . There are so many different types of chess players : recreational/casual , club , tournament , masters/titled , professionals ....
What % of the population has played chess by let's say 18 years old? And what are the average number of games they've played, having learned the rudiments of game? And what's the average ELO, I wonder?
And what's typical ELO at a park or a bar? What ELO does someone need to have to win a substantial % of the games at parks/bars etc...? What ELO does one need to have to be a respectable challenge in those venues?