Yeah, I played 1 and 2.
7 hours is if you're racing through trying to finish in a rush. I've got 9 hours on my Witch Doctor and I'm in Act 2. I've got a friend who took 20 hours to beat on normal, then there are still more difficulties.
Online only is there to eliminate duping mostly, it's really the only way to do that. The game runs many processes server side to prevent cheating. Dropping in and out of multi player is seamless, I haven't had any problems with it. You can keep your game closed to multi player if you want to play alone though.