How do they decide if a tank is eliminated or not?
The calculation takes into effect the power of the actual gun of each tank. The Type 89 the volleyball team used has terrible gun that can't even penetrate the ass of a British Matilda. The StuG III Ausf G that the Hippo team uses a gun that has better penetration.

For the Carro Veloce, their reasoning is that the tankettes are only flipped because of their small size and the computer that does the calculating doesn't register it as a penetrating shot.

As for the Maus, it's the same reasoning as question number 1. The Maus has very thick armor and regular hits from Ooarai's tanks will never penetrate.