1) We have 5 running backs on our roster in 2012. Given our new-found running game, why would we restrict ourselves to only 4 running backs. Nine receivers (not counting running backs) and four running backs seems possible, but not required.
2) I agree that if we carry only four running backs in 2013 that three should not be change of pace backs and only one a power back. We indeed require need a backup power back.
3) If Demps ends up being a kick returner and backup 3rd down back, that would fine. He may not even end up being that. Alternatively, we may decide to keep him instead of Woodhead as a 3rd down back.
4) The biggest flaw in your analysis has to do with Vereen. Belichick did NOT use a 2nd round draft choice on what he expected to be a 3rd down back. He expected Vereen to be able to be the #1 back, or at very least part of a 2-man running back partnership on 1st and 2nd down. Vereen can be a strong power back. Of course, the issue is his injury history. We shall see, but there is no reason for writing him off yet. And certainly, we should consider him a 3rd down back merely because he has been injured.
5) Ignoring Hilliard (and/or a Hilliard replacement), the team has 4 running backs under contract for 2013: Ridley, Vereen, Bolden and Demps. Belichick will decided how Woodhead or an alternative would fit in. But the time to decided is not now, at least iMHO. Belichick could surprise us and extend Woodhead during the break in the middle of the year, but I think not.