Patur Aval Assur,
I understand your argument this way:
1. Hashem in “reality” is A (corporeal) or B (non-corporeal).
2. We do not know what Hashem is in “reality”.
3. Rambam holds that we must believe B is correct, and that anyone who professes A to be correct is a kofer.
4. There were sages before Rambam who argued a type of A, and were not considered to be kofrim.
5. Therefore, how can we consider someone who truly believes A to be correct to be a kofer, when we don’t know “reality” and he has those earlier sages to rely on?
My problem with this argument is with points 1 and 2. Reality, A, and B are things only on our level. We perceive the universe to be corporeal (A), and the opposite is non-corporeal (B), but who’s to say that our perceptions are reality? Hashem is completely beyond our conceptions and conventions, so how can we make any statements using our limited terminology about the nature of Hashem Himself?
The way I understand the Rambam is that our conception of corporeality is linked to creation and modification. In other words, if something is corporeal, than it was created by something (or someone) else and has the potential to be altered by created things. Therefore, to say that Hashem is corporeal is to argue that He Himself has a creator or could be affected by creations. Would you agree that such a statement is kefira? And if we went back and asked R’ Hillel if he thought Hashem was a creation or could be changed, do you think he’d agree or disagree?