I'm late to this discussion, obviously, but it's amusing to me that a guy signed to get a look during minicamps and (if he lasts that long) during training camp has generated this much discussion.
Betts is a RFA, and he'll probably be back. He's excelled in the backup/spot duty role behind Portis, and has shown the capability to be an effective starter when called upon. The glut of RB talent available in FA and the draft this year will kill his chances to sign for a starters job elsewhere, which means he may as well stay on the team that likes him and seems to be headed the right direction.
Cartwright will almost certainly remain on the team because he too has performed at least acceptably as a backup, but more importantly he's a special teams star and Gibbs loves him for that. For Lumsden to take Rock's job he'll have to not only outshine him as a RB, but also outshine him on special teams which will be very difficult.
I have no idea what Nehemiah Broughton's going to do. I think he's on the practice squad another year - he was going to be a project anyway - and then they'll decide next year what they want to do with him.