They must have thought that both cars had stalled, and if they had, they were in far too dangerous a position to clean up under yellows. It wouldn't have been worth restarting the race either, by the time they got organised again they would have ran out of daylight.
Indeed. But isn't that excessive to throw the red flag straight away, without even considering the situation? They could've put the Safety Car first to held up the leaders and then see if the two cars can get away, to act accordingly.
I belive they weren't allowed to restart it according to the rules anyway, any red flag after 75% of the race distance means race over and full points AFAIK.
But yea, Di Resta was probably the man that deserved it the most this year. Sucks to be Spengler though, going from championship lead to 3rd in the last race...