I think one way to save Varian would be to have Velen and the Draenei get off their butts and DO something. Specifically, when Varian goes to the Exodar and does his thing, Velen teaches him from his own expierence that hate can only destroy, and the only thing to do is to keep hope alive and keep trusting in the Light. Maybe it doesn't stick at first, but Varian later sees a group of orcs somewhere, possibly Darkshore, and charges before thinking. The orcs are actually trying to stop Twilight Cultists/Burning Legion/Old God stuff and because they fail, the (enemy here) succeeds in summoning/coming through a portal. You beat them back, but Anduin gets severely injured, almost killed, and they head to the closest town to get him healed. Velen's there, because hey, Prophet, and he's able to heal Anduin because it's too much for anyone else. Varian finally "wakes up," and humbly tells Velen he was wrong, and can he please learn more. Maybe we can even get him into a Paladin this way.

It's not perfect, but the underlying point is there. I mean, we have a race that's been hounded by their former friends for thousands of years and are still the closest we get to pure good. Use them, for goodness sakes! They'd be perfect for this!