As for air sacs, there has actually been evidence in the bone structures of theropod dinasours that they had air sacs as well. Here is the summary of a paper directly about the topic of air sacs in avian ancestors. You can purchase the full paper or notice many more writings on this topic.

http://www.nature.com/nature/journal/v436/n7048/full/nature03716.html

Feathers, hollow bones, light build, bipedalism, warm bloodedness, and air sacs were all features of fast land based dinosaurs that were direct ancestors to birds. So rather than birds developing these features to be birds, these were existing features of land dinosaurs that just so happened to be able to use these features for flight as well which led to the evolution of an avian class.

Air sacs were useful to theropod dinosaurs in the same way that they are useful to birds. It keeps the weight down and helps oxygen exchange at the same time. Perhaps other bird like dinosaurs without air sacs were on a path to evolving flight but could not do so due to the absence of lightweight and efficient air exchange apparatus.

The only reason we hear about birds is because they exist as a successful result of natural selection. All of the failed specimens of evolution ceased to be and left no trace. Evolution is actually quite logical and understandable for any individual that removes the blindfold and wishes to learn.

As one of those breaks is the fact that our avian friends also have air sacks, and fly. And how would one through trial and error be able to advance with an air sack for a lung? Just a thought there, as if evolution is to work it involves more then just bones and feathers….. try looking in to intelligent design for the answer. As evolution is just that a theory.

The way I understand it is that the majority of dinosaurs did not go extinct from the impact nor lack of food. Their bigger problem was the absence of sunlight in the short term. Large cold blooded dinosaurs depended on daily sunlight for heating their metabolism to digest food matter. The reason dinosaurs were so large was because it helped them stabilize and average out day to night fluctuations in temperature because it takes a longer time for a large body to heat or cool. If food ran out, there would have been an extinction so massive, that there would be little life beyond amoeba around today. However, fish, reptiles, amphibians, bird and mammal ancestors did survive. In some cases it was their habitat or size that was advantageous but clearly in the case of mammalian and bird ancestors, warm bloodedness was the advantage. The dying plant and animal matter of less capable creatures would naturally provide food/energy for the more capable ones.

I think the best explanation for the absence of fossils at the time of extinction is because that is actually a very very minuscule percentage of the animal sample in a span of time. The fossils we find are often hundreds of thousands if not millions of years apart when it comes to dinosaurs. The sample of fossilization you ask about would only sample the individuals killed during the few day/months surrounding the cataclysm. I know nothing about these numbers but let’s say 1 billion dinosaurs died during this impact, meanwhile there had lived hundred if not thousands of billions before this, out of which we have a few thousand specimens. My point is, odds of fossilization of the quantity of dinosaurs dying at the time of impact compared to over long periods before that are pretty small and those are so rare that it may or may not be possible to find over a very long search period.