Over all I'm preferring this over the main idea. Having a separate move to block the fall damage would mean that as an airbender you wouldn't be able to charge moves wile airborn so this kind of makes a compromise on both sides. It also keeps it as a passive.
Yes but they don't have to use any special move to do so...it's more in their nature. We can assume that in game every player wants to block the fall damage. Also the only time an airbender can't block fall damage is as I stated above when they are unconscious.
See in the examples you mentioned they both received a previous blow from witch they more or less unconscious. You remember when aang was being chased by the yuyan archers ? He jumped that cliff with no concern or plan to use any move .... Or when Zahir escaped prison... He jumped of the cliff...
Even so he was a very powerful bender even before he merged with Vatuu. After his merge I believe he got even stronger and in the show and in the thread he was the only one capable of doing this (besides Korra who was the Avatar)