The issue is that "everywhere and nowhere" doesn't necessarily mean non-existence, especially since if you think about it, it's slightly hyperbolic, as he is only semi-omnipresent, not truly omnipresent, which is what is implied by saying he is simultaneously everywhere and nowhere. It seems more as if what's going on is his true self always exists as a disembodied consciousness (the nowhere part) and he can project himself anywhere he wants (the everywhere part). The state Alucard and Schrodinger are in is already covered by incorporeality. If you read the incorporeality page, it describes the nature of Schrodinger and post-Schrodinger Alucard to the letter.
Edit: I said already that the issue we all have with Schrodinger is a lack of information. That was even the excuse Rooster Teeth used to not factor Schrodinger in the Death Battle. In the old days people even used to argue that Schrodinger wouldn't regenerate if damaged sufficiently (if you think about it we only ever see him regenerating his head being destroyed), that Alucard lost all his other abilities, that Alucard returning after being erased was dependant on the piece of stone with his symbol on it and doesn't apply to his regeneration, and so on. These arguments came mostly from comic fans who hated anime, but they could make these claims because Schrodinger and post-Schrodinger Alucard have so few demonstrations of their abilities.
