For the NAD pre-amp, you would get true stereo subs, but you would also get some "double bass" on those stereo sources since the signals going to the mains (via the NAD amp) would be unfiltered, full range signals. Depending on the crossover point used and the frequency response of the speakers, this may or may not be objectionable.

For the Model 950, routing through the ICBM with the subwoofer signal should work fine so long as you don't have the NAD pre-amp on at the time. Since it will just be splitting the signal from the 950, it will be a dual mono sub signal in that case.

I assume the fixed 80Hz crossover you are referring to is the analog bass management on the 950's six channel analog input. Since the only thing happening downstream of the 950's processing (either the fixed 80Hz crossover or the digital triple crossover) is the dividing of the subwoofer signal, there should not be a problem. You'll just have to be sure only one system is running at a time, which will be true anyway since you are using the same subs in both systems.