Didn't GRRM once say that "sometimes, a cigar is just a cigar?"
I for one am pretty sure the 'retired' PoV is going to be Theon - we'll need to have him to find out what happens in the North and how people there react to Stannis, IMO. I'd understand that GRRM didn't want us guessing, but seriously, if Ned came back it'd def. weaken the entire series! Don't get me wrong, I think this is a great theory and I hated it when he died, but I think that's what made the first book so brilliant. He can't spoil the effect, albeit four books later... surely?
The other parts of your argument are that Cat and Sansa didn't recognise the remains. Weighed against that, Bran AND Rickon's dreams (on the day Eddard died, we must assume) as well as Sansa or Cercei or any of the others not recognising an 'impostor Ned' - not to mention Varys' apparent lack of motive for saving Ned in the first place, - I'd definitely say that he's dead. Sorry...
Oh and btw, Benjen was never a PoV character.