> How? "Have impact" is synonymous to "in principle observable" (...) If the sub-structure of the universe was completely screened away, so that whatever happens there, no bit of information reaches our universe, then I see no meaning in the question of whether or not that sub-structure, or entities inhabiting it, exists.

Lemme try to answer those, as this is the core of the current disagreement I think.

Consider the case of parallel universes -- they are actively studied by scientists as a notion, and there's very much a question of whether they exist, while still "no bit of information reaches our universe" from them.

Does that preclude their existence, or makes such an inquiry into the possibility of their existence meaningless? Several physicists and cosmologists (even major ones) don't seem to think so.