Wow, that episode was kind of depressing.
I was left very empty feeling at the end of the episode. I need more.
I think that was intentional, and I had that feeling until I watched it again.
When has chicken soup ever just stopped a cough like that, and the Ice King called Marceline "Gunther" at the end of the episode. Pretty obviously, she lied when she said "Marcy got better, then they both lived happily ever after." See, what I'm thinking is that Simon knew chicken soup wouldn't actually work, but when he put the crown on he forgot it wouldn't, so he couldn't give her the originally intended treatment. The fact that she is now a vampire is also kind of a hint at what happened.
I'm not gonna try to go too far with this, but Marceline said she was undead. She obviously stopped aging not long after that, so it's possible she died of her sickness and became an immortal vampire, rather than a mortal aging one. They have been covering that up well enough by doing things like making a joke whenever it is brought up so that it would most likely not occur to the younger audience, but the older audience. Well if that's where they are going with it, that is. I don't know you guys can get your own ideas or whatever, but that's what I thought when I watched it.
Also I always get Gunter and Gunther mixed up ugh why do all the penguins have to have similar names?
I'm just going to assume the spelling doesn't matter.
Actually confusing the penguin identities are probably some sort of running joke.
Additionally:

I guess this episode explains that.