Sir Donald. Apologies - point taken. I can understand your dilemma when faced with different points of view from different authorities (check pilots, simulator instructors and so on).
Been there done that and it gets frustrating when you would prefer to believe Boeing via their FCTM but are faced with convincing counter points of view from all of the above chiefs of the flight deck.
The best reply you can offer when faced with these varied authorities is to gently ask whether or not they have actually had the opportunity to carry out the measured flight tests that would be needed to prove that they correct. I doubt if they would have.
The next suggestion that you could put to these gentlemen is that it would be a splendid idea if they could share their views with the Boeing Flight test department and perhaps get Boeing to revisit the FCTM advice in order to incorporate the changes suggested by the proponents. I am sure the Boeing reply (if any)would be illuminating.
Meanwhile to be on the safe side there would be no harm in erring on the safe side and accept the Boeing FCTM advice as authoritive. Full marks to you for delving deeply into the subject, all the same.