Well, in the end, Game Freak doesn't confirm it, so we don't know.
But it's a well constructed theory that makes lots of sense - until GF confirms it, which they probably won't, then we don't know. Sorry, this probably wasn't the answer you were expecting... if it helps in some way, I believe the theory is incorrect because Cleffa wasn't introduced 'till Gen II, while all three of the Gengar Evo Line were introduced in Gen I. Not to cause conflict, just my opinion on the matter. Sorry. :3
Hope I helped. :)