OK, I guess I have 2 possible issues with Ghostly Evasion as it was originally written. One, like you say, is that it's not in the original monster. Second is just that I think it needs more distinction from the Parry ability. I think I'm ok with adding Ghost Step/Evasion, so I'll go along if we can answer the second issue satisfactorily.
I think you're onto something with the economy of actions. Let's keep the parry the way it is, so it uses AoOs and requires a roll. To distinguish the ghostly stuff, let's make it work automatically against melee and ranged attacks and some variety of spells (rays, some other things I guess) but only be available once a round and take up some of the actions. Like you suggest, an immediate action to vanish, then a required move action to recorporealize should do it.