If my two cents are worth much, the prior discussion on the various witness accounts is correct: different people saw the same events in a different light.
Mitchell's thoughts about Mucklow giving attention to Cooper I believe comes from two sources:
A. An interview in the Mountain News 12/7/14 (You are not allowed to view links.
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Login): "Why aren’t they (the stewardesses) paying any attention to me? he wondered."
B. His interview with the WSHS 10/3/13 (You are not allowed to view links.
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Loginquery=d%20b%20cooper&media=0&show=100&sort=title&page=1 but not sure if all the docs are still up): "flight attendant was paying all this attention to this geeky looking guy (Cooper) instead of talking to me."
As far as Gregory's seat, combing through all the FBI files (SE 164-81, 11/26/71), he believes it was "seat 18 which was an isle seat on the port (left) side." As pointed out here, that is not possible if Mitchell sat alone in the three seats on that side. His additional statement that he "noticed a man seated in a window seat on the starboard (right) side of the aircraft to his (GREGORY's) rear" for me rules out row 18. Thus, Gregory must have been in 17C or 16C.
Please correct me if my sources are in error.