No need to get testy, TKA. It's just a game.
Well, consider this: Anakin Skywalker got a crash course in preserving his consciousness after death from Obi-Wan and Yoda right
after he died, and I think it's likely that he would have had only a small window of opportunity to perfect it before his soul faded away like anyone else's. There's something that takes a lifetime's worth of training and discipline learned on-the-spot right there, considering he'd never known anything about it until he saw Obi-Wan's body disappear (this is confirmed). What Anakin had going for him was that he had already won half the battle by dying in complete selflessness, which is the main requirement, according to George Lucas.
That's the light side version of the afterlife. As for the dark side version, and to answer your question of when Galen would have had time to discover and master Palpatine's secret, I provided this when I first proposed the theory: during the long period of time when he served as Vader's apprentice, before the events of the first game. I'm not sure how many years he served as Vader's apprentice, but I'm sure it must've been quite a few if he was trained from childhood to adulthood. I proposed an idea (which I know won't happen) where the game opens with a prologue mission that takes place many years before Galen's death, where he is sent to discover the Emperor's secret to eternal life and decided not to inform his master of his discovery.
I'm not usually one for fan theories, but this one interests me. This whole thing just reeks of Dark Empire
all around, which the creative team mentioned served as one of their main inspirations for the plot. Yeah, it's kind of far-fetched, but hey, an even sillier theory that I thought there was no way in hell was plausible which proved to be true: when someone mentioned the whole cloning thing on Kamino.