Cooper (who worked at least 2 missions with RR),

...and hasn't seen him in 30 years...

Fitch (who knew RR well enough to say he always liked him, and call him Ray)

And where on that timeline of yours, does it show when Fitch or Kirk actually met Reddington in person?

We don't know how much contact Fitch ever had with the Real Reddington. For all we know, Real Reddington never laid eyes on Fitch. Fitch could very well have only met the Imposter in person.

and Constantin (who had a gun in RR's mouth and knew Red's voice even when he was not expecting him to be the one calling because it was Senator Diaz who did from his phone)

Same with Constantin. Just because he knew the name of his wife's lover, doesn't mean he met that man while they were lovers. Alexander Kirk could have put that gun in the Imposter's mouth, and recognized the voice of the Red, because that's the only voice he knows as "Reddington".