As far as Wrathion and Kairoz... I'm kinda suspecting this was a 'do bad to do good' in the short story Hellscream Kairoz talks about having more than just the iron horde, he wanted infinite hordes. We know how much trouble one horde on Azeroth gave the burning legion, an infinite supply of hordes could defeat the legion pretty easy if you could handle the logistics. Garrosh fucked that up by killing Kairoz but once this war is settled we could have the resources and troops of two worlds to stand against the legion at least and it shores up the horde after their civil war losses.