Here is how I see the situation. Roscoe is looking at being the 4-6th WR and the punt returnman. Right now Roscoe is battling Steve Johnson who is injured for playtime and eventually James Hardy when he returns. The problem is, there are few places that Roscoe fits in this offense. He's not a slot and over the middle guy because lets face it, he'll get killed by LBers and bigger safeties if he gets hung out to dry on a bad throw, ie Flutie getting Thurman killed against the Colts.
Roscoe needs separation to be effective and I don't think if he's on the outside he can consistently beat the bump at the line. What could be done to get him some room is to line TO and Evans up on one side of the field and try to stretch the other with Roscoe's speed. It will draw some coverage away, but I think a lot of DC's would catch on to that quick and beat it with a zone scheme.
All and all, I think Roscoe is pretty expendable, but will not be traded until someone throws a good offer at the Bills for him.
As for punt returns duties if Roscoe should leave, the obvious choice would be McKelvin, but if he is to be the #2 CB this year, I don't think i would be risking injury on PR. Fred Jackson, Steve Johnson, and Ellis Lankster would probably be used to fill that role.