I think you're giving them too much credit on Billy. I get the impression that they're doing this a bit on the fly and they don't have much in the way of plans that bridge across seasons. Billy's redemption was so rushed thanks to the complete lack of any character development. If they were planning on going that route, that could have at least planted a few seeds in season two, but there was nothing. He ultimately wound up being one step above the cannon fodder that was Gary Busey's kid this season.
I don't want to make it look like I didn't enjoy the season, because I did. The interactions between their main characters have been a blast, but they could gain from developing a few background characters as well. There's a few too many red shirts in this show. They'd probably gain from killing someone out of the main group next year (I saw this assuming Hopper will be back).