One backwards battery out of a pair will not discharge if the light is accidentally turned on. The batteries will oppose each other and it acts like the switch isn't on.

One backwards battery out of 3,4,5,6 or more won't protect teh batteries against a switch on but will just produce less light and will be a waste of the reversed battery.

Turning both batteries backward is an incandescent bulb light will make little difference except that connection might not be very good as the contacts are designed differently for each end.

I've done this for flashlights that have poorly protected switches.

(I'm an electrical engineer.)

Unimogbert