Voltage will decrease with time - close to the end of a discharge cycle, NiMH packs show a drop that is more gradual than equivalent NiCd packs. The running time will depend on the current drawn by all of the components - motor, lights, etc. Without knowing these, it's difficult to estimate how long you'll get from a pack. The higher voltage (12V) lights are not likely to be as bright as the lower voltage (6V) ones, as they are only being supplied with 6V from the pack.
A.