AceBandage
Banned
Aang did master it completely by the end though, that's also why he is the youngest fully realized Avatar.
On teh subject of Korra, I kinda agree she will not master it during the series. She will undoubtly do so sometime in her life though.
No.
He got hit by a rock on his scar and it triggered. He was not in control until the very very end when he was able to exit it, and that was not mastering it. That was him finally being able to just get out of it through sheer determination.