I tried aptitude and it works! Thank you for your help! aptitude behaves properly in example from https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=229775#66 (message 66). Moreover, stretch's aptitude version behaves properly (i. e. in the way I need).
So, my problem is solved. But I still think "man apt_preferences" is incomplete. Please, add to the first page of the manual something like this: "apt's priority-aware dependency resolution algorithm is simplistic. Sometimes it is unable to find any solution. If you find yourself in such situation, consider using aptitude, it has advanced resolution algorithm" -- Askar Safin
