BEFORE WATCHING THE VIDEOS PLEASE READ:
As far as I understood, SOME SKILLS ARE WORKING ALMOST FINE AFTER THE STUMBLE, BUT THE OTHERS AREN'T, SO BE CAREFUL IF YOU DECIDE TO RECHECK MY EVIDENCE. ALL skills were spammed with SAME SPEED after the stumble, and you can see it if you pay attention to skill icons:

You can see on this screenshot extra black stripes on the edges of the sleeping storm skill, which indicates that it is pressed (spammed very fast).
Finally, here are the videos:
1. CURSE OF ROOTS SPAMMED AFTER STUMBLE
Notaion:
Retail:
2. SLEEPING STORM SPAMMED AFTER STUMBLE
Notaion:
Retail:
These videos show that on notaion there is a delay between stumble and spammed skill, while on retail the skill cast begins right after the stumble debuff duration is over.
Torial wrote:Torial comes for the rescue!
Stumbles work exactly same as they do on retail, while in the animation of getting up, wither it's retail or not aion, you can use an insta skill (I partly believe the skill should be usable while moving too for it to work) and it will cancel the get up animation, other wise you're stuck in animation and can't move till the end, so the trick i use on my sorc is whenever i'm stumbled, i spam Robe of Flames so when im getting up it cancels it and I can move freely again.
Damn, I miss these huehue.
Thanks for the information bro, but I'm talking casting skills above all. Although I'm not sure if there isn't a bug with some instant skills, just don't have time to test it.