If you think Carpenter is a weak link, you're only basing your opinion off of the last two weeks (ie, the one game). Carpenter has been a monster in the run game all year, and while his pass protection isn't good, it's certainly not horrible. He, Okung, and Unger have been a big reason why Lynch has had such a good year (though Lynch is obviously a fantastic runner in his own right).
If you want to pick a weak link on the line, I'd say it's McQuistan, but that's almost a given - "the right guard is the worst player on our line" is probably true for most teams. McQ is fine, but relative to others he's not as good.
Our OL got blown up last game. Everyone feels bad about it. But it's easy to forget that prior to last game they'd been doing pretty well, RW and the run game were thriving, and it wasn't perceived as a huge need.
That said, I think Bevell's issues go beyond just calling him a scapegoat. There IS a point where it's possible to call too many runs, particularly on 1st and 2nd down, and it's a definite weakness of the team. There's plenty of creativity in our offense, and particularly at home it seems to click recently. But at some point his entire strategy needs to be re-thought on the road, because other than the Lions game, it hasn't worked.