Guys,
this problem is making me crazy ...

Perhaps I have done a little step forward toward the solution, or at least we can say that thrust line is not a player in the problem.

Infact, some pilots suggest to check differentials on a vertical dive WITHOUT throttle. I tried with my plane, and of course I found that even these diving rolls were good as a climbing one with full throttle...

So it seems that thrust is not an issue.