Every witness said it wasn't McCoy.
Flo, Tina, Alice, Mitchell
Every. Single. One.
During his hijacking, McCoy attempted to disguise himself, but every witness was later able to ID him.
That alone is a huge hurdle, and that's ignoring the detailed and thorough investigation of McCoy that cleared him of the Cooper hijacking.
Yet their composite drawing looked like McCoy. And the FBI office in Reno made tons of mistakes. Did not collect many items that could have been used for prints. FBI agent McCallum is a pretty smart cookie. He is very convinced it was McCoy. And one agent said that McCoy said something at his trial only the real Cooper could have known, about the money being lost. The money lost was not found until years later. McCoy had the smarts to pull it off, was tough enough to pull it off regardless of being dressed wrong. Former green beret. Flo Shaffner is a horrible witness. She has proven that. He was likely disguised before he got on the plane. How could they know? And what about his relatives saying the tie and clip were Richards? Just as damning as your stewardess testimony is non damning.
Obviously, similarity is in the eye of the beholder, to me the sketches look nothing like McCoy, especially regarding his ears, hairline, and posture. Let's not forget McCoy had a North Carolina (southern) accent and a lisp from his surgically repaired cleft pallet. It's more than just looks, McCoy couldn't even pretend to be Cooper.
Re: the fingerprints; The FBI literally removed the seat Cooper was sitting in, and had what they believed was a good palmprint from Cooper. McCoy's prints did not match any of the fingerprints collected.
SA Calume's book is actually talked about in the FBI files, and it's implied he wrote the book for money, as he should have been aware of all the evidence that effectively cleared McCoy of being Cooper.
I want to re-iterate that McCoy attempted to disguise himself during his hijacking, yet every witness was able to ID him. Cooper did not disguise himself, (unless his disguise was done by an Academy Award winning Hollywood makeup team), and every witness from 305 said McCoy was not Cooper.
As for McCoy saying Cooper lost the money, you'll have to give a source on that, I just paged through the interview chapters in the Calume/Rhodes book and didn't see that statement. Not that I trust that book... I've said this before, but there are some outright fabrications in the Calame/Rhodes book, and it can no longer be taken seriously as a source. We know this because the FBI documents are now available.
As for the tie, the FBI investigated McCoy's connection. They located two pictures of McCoy wearing his dark tie. We know this from the FBI releases. Most importantly, thanks to Tom Kaye, we know there are pin holes on Cooper's tie from the tie clip and a missing tie tack. The pictures showed that McCoy's tie did not have any clips or tacks.
And of course, we need to mention the exotic particles Tom found on the tie, particles that McCoy would not have been exposed to, and for which there is no source in Utah.
Forensically, McCoy's connection to the tie has been thoroughly debunked. It could not have come from McCoy.