FIRST DOWN BACK (pick 2)
I find it interesting that have Blount, Gray and Bolden in this position. This would mean that Gray would need to look awfully good to get any reps for Game One. Gray might make the roster as a backup.
THIRD DOWN BACK (pick 2)
As you say this position is wide open: White, Cadet, Lewis and Gaffney. I would add Bolden to this mix, since he has been our 3rd down back in the past. Bolden certainly could be the backup at this "position".
GAFFNEY
Gaffney seems to be the player everyone is rooting for, including Belichick. If Gaffney looks good in camp, it seems that Belichick will fine a place for him.
BOLDEN
Bolden seems likely to make the 53, since he can backup on all downs, and he has looked good when given an opportunity to carry the ball. In addition, he plays lots of special teams.
FULLBACK (pick 1)
You listed Develin as a running back. If he makes the team, it will be as a fullback, where he has been successful in the past. Belichick needs to decide whether to continue giving Josh a fullback as one of the 53 man squad. Some running teams do have a fullback. The position is rare for a passing team.
I guess the question is how much more effective our 1st down running backs are with a fullback. Develin was worth a roster spot with Ridley and Vereen as our running backs. It is not at all clear whether the same will be true with this year's or next year's running backs.
If there is no fullback, the position could be used for a developmental RB like Gaffney or for a special teamer like Bolden. The position could also be used to carry a 4th blocking TE who could be an H-back if needed (Hooman).