Tested and when playing.
It generally follow the same rules of all final attack skill that other classes has (only Kanna final attacks has the strange rule it cannot activate in the air).
All final attack skills description never say which skills can trigger them (e.g Hero shout/combo fury do not trigger FA, soul driver on a adventurer do not trigger FA, Aran final toss/gathering hook do not trigger FA)
Also the final attack portion of Kanna "Nine-Tailed Fury" when it is on cool down also do not activate in the air.
(A typo in "Nine-Tailed Fury" description, "When attacking while Nine-Tailed Fury is off cooldown" should be "When attacking while Nine-Tailed Fury is on cooldown"
Check the star quest left of your screen for the box for reaching lv 200.
The cash shop has nothing you can buy to extract a used damage skin into a tradable format.
You must use the damage skin extraction coupon in the event coin shop (1500 coins) to convert the current damage skin you are using into a tradable coupon.