Only Carroll is a key backman - and I guess Holland, who is not good enough to get a regular game and is on his last legs.
Rivers and Bizz are 'third tall' style flankers, and unfortunately I don't think Ferg is ever going to be a key back either. He's never really filled out and while courageous, doesn't have the strength. I haven't seen much of Warnock so I'll take your word on him as a key defender, but he's not there yet.
With Frawley injured and Miller playing forward PJ has been given game time in the backline. I think that's a decent move, because - while I agree he's probably been a forward historically - he's not going to be an AFL-level ruck IMHO.