Thanks for the feedback - always interesting to see other people's thoughts on complex subjects.
In regards to the hypnotism, I'll try skimming through the early eps containing Marceline again at some point to see if I glanced over it.
As for "Marceline's Closet" - you're right that the lyrics actually do contain the first mention of vampireness and immortality taking its toll on Marceline! I had to go look up the song lyrics themselves because it's hard to decipher them from listening to the audio during the episode due to all the other sounds going on. I knew that Marcy's history with IK was likely planned around S3 due to her journal having "Gunter" on it and all, but I had never actually looked up the dampened bits of her lyrics in that episode before. Were there other little hints I missed during later seasons (4, 5, or 6) or was this the only one? Great job pointing that out - wish I had you to refer to before I completed the video, haha (I actually asked a few other peeps if they could recall whether Marcy had behavior in prior seasons that established her being tortured by being an immortal vampire, but like me they couldn't recall any).
My main problems with early Marcy isn't about her being mean (and I actually praised the facade aspect she has), it's specifically about the ways in which she played cruel jokes - with the insinuation that she was turning somebody into her puppet via vampire powers. Stakes established that sort of thing is basically a trigger for her, and I even after hundreds of years I cannot see her doing these sorts of things as jokes (soul-sucking jokes and other stuff on the other hand - that's cool, those sorts of things actually allowed her to emerge victorious in the end and save people).
Like I said in the vid, I liked Marcy giving up a lot after I thought about it; I saw it as Marcy rationalizing her way out of the fight due to fear, but not actually wanting to voice that she was afraid to continue her struggle so she tried to place the blame elsehwere. But there are a ton of different ways to interpret and analyze this scene I'm sure (which actually makes it a whole lot more interesting too).
And yeah I've been reviewing Adventure Time for a little while already - I got all the eps leading up to Stakes reviewed (and I did reviews of S1 eps when starting out but the early ones are mediocre IMO). I'm gonna skip to "King's Ransom" for my next review since I'm a bit behind already (and I usually don't enjoy hijinks-based episodes as much ones that tie into major plotlines, so I'd rather do what I have more fun with first).