You're right pondedaniel.
I never heard anybody say "mayn't"
I guess "may" is a disappearing word from the English Language and the substitution of the auxilary modals as suggested by Henry Cunha provides common usage.
I would substitute the second example with "could"
e.g. You and me may get 10 on the test, may we not?
You and me could get 10 on the test, couldn`t we?
(or better You and I
could get 10 in
the test, couldn`t we?)
The construction "could we not" is also not common nowadays and can appear after the original modal
e.g. You and I could, could we not, get 10 in the test.