It's doubtful that bacteria would get through your airlock as long as the airlock itself was clean. Pasteur's swan-necked flasks were essentially s-shaped airlocks with no liquid in them. Those flasks remained sterile for years.
The oxygen is the main concern, but I don't think you have to worry.
CO2 is denser than air and your yeast are most likely still producing it unless fermentation is finished. As a result, the CO2 will sit on top of your beer and keep the Oxygen from getting in. If the only opening to the atmosphere is through the small opening of your empty airlock, then you probably don't have very active gas exchange.
As for the question of why yeast need O2 early on--the main reason is because fermentation is less efficient at harvesting energy than cellular respiration. Without going into the biochemistry, respiration harvests between 15 and 20 times more energy than fermentation does. As a result, providing yeast with oxygen early on allows them to get to high population density earlier than if they had to rely on fermentation. This high population allows them to outcompete any other organisms. Once they have scavenged the dissolved oxygen, they shift over to anaerobic fermentation which produces ethanol and carbon dioxide.