You are correct that it's south Wales where the majority of the population in Wales lives, so you could well be correct that rugby would be the most popular sport in Wales, full stop, but I still have my doubts. Rugby is the national sport, but that doesn't necessarily translate into dominant sport. It also depends on your friends, and the places you go I suppose. Like you, most of my friends from back home watch and follow rugby, although some religiously follow club football as well (it's pointless talking about the national teams, because they're rugby dominated in every way).
You're correct about the local papers, but they aren't big selling rags. It's still the big papers from England (Times, Guardian, Sun etc.) that sell the most in Wales, and they're all football. I'd like to know the viewing figures for Scrum V in Wales, compared to Match of the Day, that could be the thing that would settle this debate, but I'm not sure if they release figures for individual areas. I've found an average figure of 300,000 for scrum v (on an old 606 thread, so no idea if it's accurate), which is not quite 10% of the population, can't find anything regarding motd etc.
The big cities in South Wales, Cardiff, Newport, Swansea would have a big bearing on the outcome, and I'd still wager that football was by far the most popular, especially in Cardiff and Newport in terms of club viewing attendances (both live and tv).
Yeh, my focus on Aber is probably a little narrow, just trying to use examples of what I know best. I'd like to hear other peoples views to see if we can build up a better picture.