MMO games don't take a lot of bandwidth in most cases, and typically around 16MB per hour is what you're going to be looking at, some games may pull 20-25, but even that is pushing it. I haven't tested this game in particular, but assuming that it uses under 18MB per hour, which it should, you're looking at about 50-60 hours of gameplay per GB, depending on exact values.
1.) Most likely, less than 18MB per hour.
2.) This doesn't matter much, to be honest. Even if YOU aren't doing things, things are still happening around you due to NPCs and other player characters.