If I have 18+ green AP and lay down two One Fell Swoop countdowns and goes airborne, when he returns, will it do 2x damage or just the one countdown’s amount?
Pretty sure they both go off one after the other
Each countdown deals the damage stated; their condition is evaluated separately so both will go off as the condition (“Vulture returns from being Airborne”) has been met.
Each countdown tile effect takes turn beginning from the top left tile all the way to the bottom right tile. Since each “One Fell Swoop” occupy a different tile position, you are actually dealing damage once for each One Fell Swoop. This applies to all other tiles as well, unless they stated specifically otherwise, like 4* Starlord passive ability.
You do double the damage, but only one animation.
*zero animations 
If you are brought down by Archangel, does it trigger once the stun is gone?
No. The stun prevents Vulture from removing the countdown and dealing team damage.
5* Thanos’s purple can also neuter Vulture, since it prevents him from removing the countdown tile. This is true for both a friendly or enemy Thanos.