So, if I understand it.
Simon is an Archaeologist, he finds the Enchiridion and, presumably, the Crown.
My personal theory is that the discovery of the Enchiridion introduced Magic to the world and led to the Mushroom War.
Either way the Mushroom War happens, Simon, with the crown, finds Young Marceline (We don't know where Hunson Aberdeen is at this point, probably doing something Chaotic Evil).
Simon raises Marceline. He uses the Crown to extend his life (Since she ages very slowly and can't survive on her own yet)/ give him the power to defend them from whatever threats are wandering around in the ruins of the Mushroom War. However, continual use of the Crown drives him insane. Eventually Simon: Archaeologist, Musician, and devoted caretaker is gone, and all you have is the Ice King. A sad man in a frozen castle with some penguins.
Also, I now want to see more of Simon & Tiny Marceline.