I guess I'll go back to Occam's Razor. I just think the simplest, most logical conclusions are more likely to be accurate.
Hawk and Barnett just somehow "forgetting" how to be linebackers: Makes no sense, so I don't see it.
Injuries: Barnett's is obvious, but I have a hunch (admittedly it's nothing more than that) that Hawk's been more banged up than we've been told. I think he's been hampered by stuff most of the year, and then moving him to the middle made no sense - which leads me to...
Coaching: Possible - I think we saw a lack of creativity and flexibility in a lot of our coaching this year, and Moss could be in that group.
Scheme: This scheme depends on stout D line play, and has since Bates ran it in Miami. It's not like Zach Thomas was shedding offensive guards or anything. This rolls us back around to...
Injuries (pt. II): Losing Jenkins was HUGE, as was KGB's inability to round back into form from his knee problem. We just weren't deep enough on the D line this year to absorb those two injuries, and our D line play suffered horribly. Since the scheme is heavily predicated on the play of the D line, those injuries also affected our LB play.
It's a combo platter, folks. 1. We sustained critical injuries* in crucial areas of our defense, which cascaded along said defense. 2. Our scheme is either not flexible enough to allow for the adjustments necessary to make up for the injuries, or 3. Our coaches don't have enough of a handle on that scheme to make those adjustments.
That explains it well enough for me. The good news is that it's totally fix-able, without having to make wholesale changes to our personnel (on the field; I think changes in the coaching staff are warranted).
*I know, I know - "injuries are no excuse." Gotcha. Except this isn't Madden where you can turn of the salary cap and force trades, and this isn't the preseason, where you can have 80 players on the roster. We lost crucial pieces of our defense to injury this year and didn't have the depth to account for it. That's party on the FO, but it's also just how it goes. The Giants were/are insanely lucky that guys like Tuck were able to answer the bell for them, but I firmly believe it was just that - luck.