Cormier played it safe against Mir but there's no reason to believe that he wouldn't take down Nelson. Nelson has 0 sub victories against high level opponents, Mir was coming off breaking Nog's arm. There's also nothing Roy could do if Cormier wanted to dirty box him against the cage like he did to Mir. To reiterate, Dos Santos, Werdum and Mir all beat the piss out of Nelson. You don't have to KO someone to beat them on the feet. Nelson has a lot of KOs but his striking isn't as good as you seem to think. He has an overhand right and a chin, so he always has a chance, but against elite opponents that's not enough.
No, I suspect Cormiers striking isn't as good as you seem to think. I don't think he'd be able wall n' stall against Nelson like he did Mir either. I just don't see Cormier winning that fight... and I'm no Nelson fan.
Also, didn't Nelson destroy Frank Mir in some BJJ tourney years ago?