I'm not as deep into the history of the games as you are, so I can't understand your arguments here (and therefore can't refute them or suggest a solution for them.), although I believe they're valid.Except again, you have the dilemma of this going against the events and rules that were established in the past game and the author's insistence that they share the same universe. Had Cari kept the shared universe thing a secret, then the sci-fi implications of the story could be taken into consideration without issue. However, the shared universe thing (as well as the secret ending in OIAL - which establishes that OIAL is just some sort of "server" to Eternum - being clarified by Cari as non-canon and just a fun easter egg of sorts) kinda makes that difficult unless you throw all of the established rules of the setting under the bus.
But the fact that Axel said the computing power on earth would not be sufficient does not mean that there are no data centers on earth running Ethernum. So also a server, or server farm, for OIAL.