The phoenixes (phoenii?) don't despawn during gravity lapse, they just become inanimate and don't do any damage during that time.
Also, the nether vapor isn't necessarily something that is a set distance from Kael'thas. There are pink clouds that float around (they're kinda hard to see because of the color of the room), but if you look directly at the floor you should be able to see them (they usually don't stray too far from where Kael'thas is standing). I usually stay about 20 yards away from him until he starts casting his first fireball, then I run in.