>first paragraph
Existing beyond exists, well, beyond, and should be tiered that way. A non-layered existence isn't an AP feat by itself. This is really the same as "beyond dimensions" and "adimensional" except with layers instead of dimensions, and should be treated as we normally treat those feats.
>the rest of the post
I think you're strongly misunderstanding how this will play out.
- When realities are equalized, characters get their key within those realities.
- Realities are not always equalized. Sometimes characters have keys for how they perceive that reality from on high (SCP-3812), sometimes characters have keys for how they perceive that reality from down below (SCP-2747).
We are not placing outside characters on a specific layer.
EDIT: Unless you had, for instance, a character beyond all outerversal structures (High 1-A) fighting a character in a verse where beyond all outerversal structures there's more layers, in which case this sort of thing is unavoidable. There's boundless characters down in tier 2, but we treat them as bounded by tier 1 cosmologies.