Life as a journey, you are correct in that the 767 does not have a mach tuck problem where as the 757 does require the trimmer. The aerodynamics of it all are lost on me now as it has been some time since I flew both. I will dig out some old notes and see if I can find out further for you but bear with me as the roster is heavy this month.
Just a thought, there is a website called "bluecoat" where you could post such questions of a technical nature and it would be answered by Boeing/Airbus engineers. Initially the site was set up by NASA and then expanded there on to encompass civilian aviation. Whether it still operates I do not know but was of use when I had questions that the manuals did not cover during the rating and subsequent flying. If you managed to find it then please send me a message with the link.
Cheers.