Well some stuff came up from rewatching the episode and reading the wikia:
-Ice King subconsciously has an attraction to come to Marceline. Part of the reason he likes her music so much is due to his subconscious [simon] knowing who she is and always wanting to find her to see her again. After Betty left him, Marceline is probably the closest he came to loving someone like family.
-He said the pages smelled like wet tears. This is implying he has at some point, or many times before, regained some knowledge about who he is and what he has become, seen the scrapbook and started to cry over the memories/realization. Then he forgets again [Alzheimer's and Dementia].
-While Marceline says she moves around to try and avoid him, she also sang in the earlier part of her singing that she's always glad to see him when he DOES find her again. Similar to dear family members with Alzheimer's. You dread seeing them to realize they don't recognize you but when you do all the good memories come flooding and whether they recognize you or not, it STILL is the person you care about.
-When he pushed Marceline away, he showed more remorse about simply pushing someone than he does about doing something that could be considered worst to someone else. Like removing body parts from princesses, or freezing Finn and Jake so they spend time with him.
And yeah I agree with you Jack. Hence why I want to see an episode where they show glimpses of their journey and him losing his mind, or an episode where Simon escapes the madness for a second, says something to her, then loses it again.