Once you completed the quest to unlock skip to the last boss! It should be account-wide so you dont need to reclear all on alts!
Thank you!
It already is warband wide,at least in Nerub’Ar so I´m not sure exactly what you´re asking for…
I only take it on alts for the gold, because they´ll likely eventually also get their 3 council kills, and 40g is 40g… 
I am talking about to farm mount on all characters and you have to do full reclear if you havent done quest on any alt.
It doesn´t matter why you want to reclear, it only matter that you do.
Again, I´ve turned in the quests for N and HC skips exactly once, and yet still offer and can unlock skipruns on my alts.
So either we´re not talking about the same thing, or something else is wrong here…
I dont know about normal/heroic coz I am doing mythic. I couldnt skip one of my alts so I have to do quest skip 4x reset.
Ehm, You are aware that both the Armory and raiderIO are public, and that anyone that wants to can see exactly which bosses this toon (and in the case of mythic account) has killed, and how often, yes? 
But ok, then the mythic skip might work differently,I couldn´t say one way or the other because I also don´t have the 3 court kills to unlock it.