I don’t know what the hypothetical possibility of a navi greater than MRA”H has to do with this.
That is hypothetically possible, just not true. It it not possible, even hypothetically, for Hashem to have a guf (although this is a side point). And, no, I don’t need to prove it, and I haven’t, but I need to reject any other possibility in order to fully believe it.
Hypothetically and possibly are two different things. I could hypothetically be a billionaire, but it’s not possible that I am. As far as I’m concerned, it is possible that you are.
Back to Hashem’s non-corporeality; even if it were hypothetically true (and I don’t think so, because Hashem’s essence is what it is and cannot be different), I must believe it’s not possible to be true. Once I accept the possibility, meaning the legitimacy of another opinion, my belief is not “sheleimah”.
I think tanur shel achnai is actually useful as a contrast. Even if there is an “actual reality” in terms of “klapai shmaya galya” (and in a sense there is; I’m kind of nitpicking on the terminology, and I think it’s an interesting discussion on its own, but I’ll agree not to disagree, at least for now), there is no chiyuv to believe it, so I can accept the possibility that klapai shmaya galya it’s one way, but since lo bashamayim hee, we practice differently.
When it comes to the halachic aspect of “Who is an Apikores?”, it’s the same, the problem is that it’s also a chiyuv to believe. I could accept the notion that halacha could dictate that I treat someone a certain way despite a “klapai shmaya galya” to treat him differently. However, in the realm of belief, it is simply impossible for me to accept that Hashem wants me to believe something which is not true.