This sounds crazy, but I think Edelman actually has the best chance of being a FS for the Pats next season of all the guys you listed. He's got decent speed and size for the position. He was a QB in college, so he's got experience reading what is going on in front of him. We know he can tackle. He was smart enough to handle a college QB playbook and WR and CB at the same time in the NFL. He's got a defender's mentality on the field. It's just a matter of putting it all together.
Dowling was playing well before he got hurt, and the fact that he beat out both Bodden (who was healthy at the time) and Arrington to start despite a lot of lost time during the offseason says a lot. And I think he's exactly the kind of physical corner that Belichick wants starting.
Arrington seems at his best when he can smother a WR at the line of scrimmage. The deeper he is in coverage, the less I trust him. If you put him 20 yards off the line of scrimmage, I think he loses a lot of value.
McCourty should be back to at least close to his rookie year level of play at CB. They've got a whole offseason to figure out what was wrong and fix it.
The fact that Moore was cut in favor of Slater at safety says a lot about his future there. I think he's got long-term #3/4 CB value, but not much value at safety.
And I think that's absolutely ridiculous. You can read why here:
http://www.patsfans.com/new-england...3-man-projection-draft-needs.html#post3011683