It is incoherent to say a maximally great being could be surpassed. A maximally great being defines the highest possible limit. It isn't arbitrary; it's the law of excluded middle. Either a maximally great being is maximally great or it is not. If something else can go over a maximally great being, then it was never maximally great to begin with.To what I put in bold: Exactly. In short, something that a person believes is "maximally great" might not actually be. Therefore, it is not incoherent to say that a supposedly "maximally great" being could be surpassed; it just means that the person describing it as "maximally great" was incorrect. Your argument is apparently based on the idea that once you define something as "maximally great", it remains that way. However, this cannot be guaranteed, because your argument is solely based on logic, which cannot contradict reality. For example, the speed of light is believed to be the maximum speed that anything in the universe can go. It is the "maximally great" speed. But all this means is that we know of nothing that can go faster than light. This is no guarantee that we might not discover some way to surpass it in the future. The same, of course, applies to your argument, but more so, because there is no evidence of the "maximally great being" that your argument requires.
He doesn't actually have to state what the maximally greatest being actually is. I can say that Rudolf the red nose reindeer travels at the maximum speed. You might assume that is the speed of light, but if RTRNR travels at C2
in hyperspace, that's entirely his business, and I don't have to know anything about it.
It can be shown very easily that the Christian God is not the maximally greatest being, because he only has one begotten son. If he were maximal, he'd have 2, seriously. How's he going to save people on other planets, and in other parallel universes. If I imagine the maximally greatest being, he seems to be significantly greater than the Christian God, because parallel universes and other worlds are not mentioned in the Holy Bilge.