Dinosaurs, being warm blooded, needed a minimum oxygen level. The avian dinosaurs had evolved bigger lungs to cope with the increased oxygen demands of flying, and so were better able to cope with a lowered oxygen level.
How else to explain dinos all dying off, on separated continents, even dinos that spent all their time in the ocean?
I agree.
A meteor/comet that large would punch a large hole in the ozone, and allow the atmosphere to escape into space. This would dilute the content, besides the other chemicals and gases that the meteor/comet and the explosion would ADD to it.
Death would probably come slow, with the creatures becoming erratic (losing their minds) from the lack of proper breathing atmosphere. Of course, this would all be dependent on their distance from the impact site.