Re: Mobian Lore
1) Given the small size of the craft, I'd say he was in space to achieve a higher speed to get from point A on Mobius to point B. I.e. he was turning what would be an 11 hour or more flight with a modern aircraft into maybe an hour flight. After all, if craft in space can circle the world in around 90 minutes, and you only need to go round half the world, assuming only 10 minutes to take off and land you can go from one side of hte planet to the other in just 65 minutes or just over an hour. However it would be quite possible some criminal element in the galaxy have found the planet and would use it as a refuge or if they do business with Robotnik on the black market (which would also keep the 'common masses' in the galaxy unaware that the planet even exist), he has at least enough sense to not let them realize he can roboticize living beings as Robotnik wouldn't risk a) them trying to steal his technology or b) chickening out and informing the space forces or some equivalent group that would currently have the power to stop him.
If Robotnik does deal with alien begins he does so in secret as he doesn't want the FF groups to find out as they could try to mess things up as he's already walkign a fine line by dealing with criminal beings and lieing to them about how he creates his robot workforce. This is actually hinted at in the episode where Sonic and Rotor head onto that low orbiting sensor/spying station (Skyspy) and there is that seemingly organic tentacle monster they encounter which I doubt was native to Mobius, but it could have been a robot but it just didn't look like one. The problem with just the terrans loosing the data about heir home world while other species interact with it, it doesn't take much for another race to realize mobius would be earth and then use that as leverage in a deal with the terrans. After all its not just Terrans that don't realize mobius and earth are the same planet, its pretty much the entire galaxy, heck I doubt if even Robotnik realizes Mobius is Earth, yet, since in one episode he says 'something is rotten in Topeka (sp?). Where ever that is.' which indicates that terrans/overlanders know of earth and know some things about it, at least phrases that have endured, but have lost the history and geography of the planet from prior to WW3. The few criminal elements from alien races that deal with robotnik wouldn't know that mobius is earth, just that mobius is a place they can do business on the black market due to how remote it is as they discovered it after it became mobius.
2) Actually with the technology we have now, it is possible to build geenration ships that could travel to other stars, the only thing stopping us is the cost and praticality. Given current attempts to commercialize space, I'd say in as little as 100 years space travel could be profitable enough (simply due to resources in the rest of the solar system) for companies to have large space craft on hand for various stuff, and it wouldn't be hard to gather enough ships and stuff for a mass exodus.
And regarding the 'not a question': Civilizations tend not to last for thousands of years, but think of how long China's and Japan's history goes back? A nation and group of people can outlast a form of government. It's quite easy for people to forget, but with digital technology around, something would have to occur to wipe out that form of retaining knowledge, and a big war could easily do that. So your quite right about other nations existing around the time of Naga, and it is quite likely. I'm guessing the massed exodus occured in a hurry, so the people who left didn't have time to gather stuff to take with them that wasn't absolutely essential for life. (So no copy of the 'current' historical records.) Now it wouldn't just those that can't fight taking part in the exodus, but people who are trying to stay out of the fight as well.
Of those that survive the war, so much of the infrastructure was destroyed in the war, the survivors would clump together but quickly things would become historical legends, so the humans on mobius would know that they are native to the planet, but their knowledge of the past would be no better than the Mobians due to many factors, all the knowledge destroyed in the war, people trying to reject what they do know simply because they didn't want to believe it. Additionally, for several generations immediately after the war, people would be more focued on surviving than preserving historical knowledge. That said Overlanders would probably use technology a bit more advanced than Mobians use, which is why when the Overlanders fought with the Mobians, it was only when Julian (aka Robotnik) switched sides and gave the Mobians advanced overlander technology, that the mobians were able to win the war, as the mobians had no realization as to why robotic soldiers that don't need input from living people to make a killing decision is such a bad idea. (Which is why overlanders don't use self aware robot that can operate as independantly as SWATBots, as their loyalties can change due to a programmer. Now remote controled and assisted controled robots would be common place as they are even now being used by the military.
Also, as said in the movie 'The Man Who Shot Liberty Valance (1962)', "When the legend becomes fact, print the legend." Which occurs quite often in real life and historians have several examples of where what the common masses have been taught, isn't what really happened, yet people still believe what has been taught. So there would be plenty of buried relics that are now doubt damaged that give clues to the past, at least for those who know where and how to look. Historians would know that prior to naga and its contemporary nations, there was a massive global war, but not much would be known prior to that due to the destruction of so much. yes it may be possible to find a painting or some other form of art that predates the war that somehow survived, but most of the knowledge would truly be lost forever, which could explain why the time stones were created to try and relearn what was lost. But that 'technology' was no doubt abandoned due to the dangers involved with messing with the timeline.