I can't really comment on the first few questions as I myself have only been playing for about a year and a half... but as for how popular Airsoft is in Europe and other places:
I live in Italy right now and used to play paintball a lot back home in T.O. When I moved over here, I tried looking for paintball places again, but they are pretty rare. Mostly all up north near the American Airbases.
Then one fateful day I walk by this shop with gods arsenal of airsoft weaponry and gear ! One thing lead to another and I met the guys I now play with every Sunday. Its massive here in Italy and else where in europe. I myself have attended a few games where there were 300+ players invovlved. Every region has its own club and they are, believe it or not, very observant of rules and regs. And as for shops, there are three or four in Rome alone and a really good retailer in San Marino as well. Tons more up north (thats where all the money is!) : )