I wonder if the very low density (relatively speaking to today) make them more robust against gamma-rays and other radiation problems once outside the atmosphere?

if I remember correctly, and it's been decades of course

four of the computers ran in parallel with the exact same instructions in case one failed or came up with a wrong answer

and the fifth computer was the "decider"

is that understanding correct?

ah I see now you mention

     Eight networks were assigned to flight-critical systems, 
     with each CRT display and engine controller connected to four networks for redundancy.