I have a question!! I vaguely remember that moments before Rose entered the game, she noticed that many other people were playing Sburb at the moment, which caused meteors to frantically rain on Earth, destroying it. Is it safe to assume that there might be other humans who managed to enter the medium but in other sessions, unconnected to that of our 8 main human kids and, therefore, unbeknownst to them?

Yes, although based on the walkthroughs written by others playing the game a lot of them never entered.