I understand that Superboy Prime, pre-FP Henshaw, or some version of Superman Red/Blue are the common theories, but I don't understand how they are at all consistent with Clark's disappearing trick. Something's off there. I think that fully rules out Prime for me. It also hints at Clark's existence being somewhat flimsy/transient. Maybe he only exists in the minds of people Supes has a connection too (Lois, Superman, Lex, Daily Planet workers....) and only exists when observed? IDK the disappearing act messes with so much. I'm leaning towards a) Mxy in some form. Could have broken out of Oz's prison and then travelled back in time (5th dimensional, remember) to where we met him. b) Some form of Matrix. c) Some weird energy/reality warp figment of Superman. Really I don't have any great ideas, but the doorman not seeing him seems to fully rule Prime out.