I am confused. When 
ults a champion (or champions) he appears to be airborne himself. Does this mean an enemy could ult him? What happens when multiple ult at once? Just seemed like that this would be buggy when "all for one" comes back.
While Yasuo is indeed in the Air, Last Breath specifically ignores champions who are visually in the air due to their own abilities, such as the case of Rocket Jump, Leap, Leap Strike, and so forth. This is to place more pressure on Yasuo's ability (and his team) to knock their opponents up.
As for multiple Yasuo ults, I'd imagine that each Yasuo ult would attempt to extend the counter by an additional second, but then you'd have a full 5 seconds of additional air time.
