Well I have a theory. But first, here is the chronology of important events: Note that years are denoted as "Age" with "Age 0" being the Mushroom War. Also, note the time and date this was written; it is before the airing of "The Lich."
Age -5: Simon Petrikov finds the Enchirideon, Simon Petrikov buys the crown from a Scandinavian dock worker. He puts it on and begins to lose his sanity and loses Betty, his fiance.
Age 0: The Mushroom War occurs and wipes out all humans with the known exceptions of Simon and Marceline. Simon meets Marceline and gives her Hambo from the ruins of a toy store.
Sometime between Age 0 and Age 1000: Simon completes his transformation into the Ice King, the Kingdoms of Ooo are established and the magic returns. New species are created and somehow Finn comes into existence. The Enchirideon somehow finds its way to its new location. Billy's adventures occur and the Lich King is defeated the first time.
Age 1013: Finn finds the Enchirideon, Finn defeats the Lich the second time, the Lich possesses the "waving snail" and Princess Bubblegum, the Lich gets the Enchirideon, the Lich returns for a third time, Marceline attempts to restore the Ice King's memories.
Now to try and explain the Ice King's behaviors. I believe that Betty became the Princess Bubblegum after the Mushroom War. Basically is my theory is based on the wording of what Simon said. All Simon said was that he never saw her again after the hallucinations started. I take this not as she couldn't deal with Simon's curse but instead wanted to become immortal like him so she went on a search to find another similar crown. The crown that she found was the tiara that Princess Bubblegum wears. The crown created a similar effect to the glasses of nerdicon and made her intelligent swapping Simon and Betty.
Here is how it all fits together under this theory. Princess Bubblegum is scientifically inclined in her character design because Betty's intent is to rid Simon of his curse. However, Betty lost her memory and is unable to remember that. The Ice King is in love with Princess Bubblegum because of the fact that, since she was at one point Betty, she is "his princess." Marceline and Princess Bubblegum have the rivalry because Marceline has abandonment issues and wants the affection of her father who didn't abandon her willingly, Simon. The Ice King wants the love of Princess Bubblegum and is indifferent toward Marceline in his current state so this creates Marceline's feelings toward Princess Bubblegum.
I also believe that Hunson Abadeer is not Marceline's real father OR he found the locket and became the Demon. The difference between him and Princess Bubblegum and the Ice King is that he would have retained his memories (he remembers that Marceline is his daughter) and he made her a vampire so she would live forever. The option that he is not the father is that Simon was her father figure until he became the Ice King and she was taken in by Hunson Abadeer as a little girl.
Hopefully tonight's episode can shed light on some of the mysteries.