Please check out www.volcbat.com. We take on the role of a fictional British light infantry platoon, doing co-op missions based entirely on the sort of things an infantry platoon would do.
We fill about 80% of your requirements; we play twice weekly (theres no attendence requirement, either), we're running a campaign atm - so there's your cool scenarios, plus theres always guys playing Domination or other games together on none mission nights. You won't get to act as a sniper, but you'll get to take on different roles within the section, and theres possibility to progress to our Patrols section - sort of a specialised recce unit that occasionally offers marksman support. We've years of combined real world experience, so you will get to play in an infantry role, rather than a 14 year olds view of what constitutes an infantry role. I'll be honest, it's not that much variety, but we've been doing it since late 2003 and are still hooked on it; either we're sad bastards or we've got a winning formula.
We don't mind people not being too good - it's attitude and teamplay thats important to us. We do try to play realistically, however, we do realise we're not the internet army, and there is no need to ever call anyone Sir - my signature should humourously sum up our attitude to that!
For more info, check our forum thread:
And info pack: