Every question answered. Yes, it would be a civilization ending moment, but that moment is likely going to occur long after humanity will have no fear from it.
http://volcanoes.usgs.gov/volcanoes/yellowstone/yellowstone_sub_page_55.html
Does not list either the Siberian or Deccan Traps events which released 10 times the amount as the largest Yellowstone event over a period of 30 centuries ... covered millions of square miles to depths of hundreds of feet.