[[WMG: All of the Beyonder's origins are true]]
Ok, we can assume that the first origin story is true, and the Beyonder was truly one of the the most powerful beings to exist in the Marvel Universe--the sum total of a multiverse. This being desires to understand desire. A being on the Beyonder's level of power would be easily capable of time travel, reincarnation, and splintering of his life-essence into several different beings. In trying to understand mortals, he could easily have traveled back in time and become an Inhuman for example, or allowed himself to be "convinced" that he was a Cosmic Cube and become a part of Kosmos. All part of understanding what it was to be mortal and to desire. I'm not saying that he was faking his apparent lack of power, but everytime we see him as weaker is after his apparent death and resurrection. Most of his former power could have been expended in that act.
[[WMG: The Beyonder was always an inhuman]]
The Beyonder was retconned from being a nigh-omnipotent being to a ''vastly'' powerful inhuman, but maybe he was ''always'' an inhuman. It is possible that the Beyonder's power was greatly reduced by the [[TopGod One-Above-All]] as a punishment for trying to destroy the Marvel multiverse. This event would render the "retcon" as a lie told by the writers.
** If he was always an Inhuman, then why in all previous appearances that he acted like a sentient abstract force of energy that had constantly had to strive to understand what being mortal was like. If he was really an inhuman all this time wouldn't he just remember what it was like before he got his powers? He didn't even have a body until he copied Captain America's body and didn't even know how to go to the bathroom until Spider-Man taught him? Was he just trolling everyone by acting ignorant? Another point is that Terrigen Mist of the Inhumans never demonstrated giving powers even close to what to the Beyonder was capable of doing: casually destroying galaxies, tossing around Celestials, and eliminating Death itself. Even the Beyonder's weakest incarnations are several orders of magnitude stronger than the mightiest Inhumans. And even if he was an Inhuman, why would he bow down to Black Bolt, a being who is infinitely inferior to him? It's explained in the comic that the Beyonder remembered that Black Bolt is his king, so he remembered that fact, but didn't remember how to go to the bathroom? The Beyonder once mind-controlled every being on Earth and forced them to bow to him in Secret Wars #3, and that would have included his "king." The numerous problems with the Illumnati's "retcon" of the Beyonder goes on and on, and it's much simpler to simply ignore all that occurred in that issue, since the author of that comic apparently decided to ignore every other known fact about the Beyonder.