What you came up with is inaccurate, then. Naturally [i]I[/i] know what decides upon an Arcanist's magus, but it hasn't yet been divulged because it's a plot point to come. I can however tell you that there is no inherent link between an Arcanist and a hypothetical vast omnipotent universe. Magus is something specific to Luxas and has an origin point. The long and short of it is that magus is demonic mathematics, it's knowledge from The Nocturne, which itself is a vast unknown. The reason Arcanists can draw power from The Morrow Realms and the planet itself is because The Nocturne helped establish two of the realms, and exists in the shadow of the other.
What [i]decides[/i] upon an Arcanist's magus is not something that can be appropriated. You've said that the information isn't inherited, just the ability, and that Ibyxx learns through observation, but that's akin to learning how to speak Russian through lipreading without a dictionary: You have all the tools you need to speak [i]English[/i] (and use your [i]own[/i] magus) but the language they're speaking is an entirely different one, the internal processes invisible and the point of reference basically non-existent, the only thing you have to work with is that both of you are definitely speaking [i]some sort[/i] of language. And again, I've made it that way specifically so nobody can use the "every power" model.