I'd take the locks affliction and the priest shadow for levelling personally, once you have a little experience and gear, you'll be able to dot up a mob with instant casts then ignore it, moving onto the next one

As for nightfall you might be able to add it as a buff that shows up on the unit frames (such as pitbull) on your main toon. I'll have to test this at some point as I'm levelling the same trio (slowly) I might end up speccing the priest disc/holy for lvl 70 pvp though. Warlocks will likely be Soul Link/Siphon Life for survivability