Could Mr C be the real Coop?
Interesting thought but I don't think so.
We saw Coop going into the Red Room and DoppelDale coming out (at the end of Season 2). We saw Laura talking to Coop 25 years later and him leaving the Red Room (via the Purple Place and plug socket) at the start of Season 3.
Dougie has been exhibiting characteristics of Coop (love of black coffee, cherry pie and his FBI disarming techniques). The narrative is set as Dougie-Coop journeying back to Twin Peaks and his old self. For example we know Naido is important and Naido met Coop when he was in the purple place, she didn't treat him as a threat then.
I think where Phillip Jefferies came from DoppelDale may have already existed (experiencing time in a non-linear way as we experience it) and his statement was probably indicating this.
The Dale Cooper we saw throughout seasons 1 and 2 was clearly mostly good. Of course, nobody's perfect, and I'm sure he's done some things in the past that he regrets, but he's mostly a very decent guy.
The Dale Cooper of seasons 1 and 2 would not have just gone round killing people like Mr C has been doing. I mean, he even resisted temptation when Audrey turned up naked in his bed!
A thread from the Episode 13 forum discusses this.
Plenty of stuff to consider there.
Coppula eam se non posit acceptera jocularum