Just a few thoughts.
My first impression upon re-reading the notes after some time was that this may have been an individual struggling with their sexuality or gender identity. Doe's words just seem to reek of self-loathing. I found it interesting they referred to themselves solely as "a person" rather than using any gendered language. Maybe 'he' felt more like a 'she'? I really do wonder, especially since they said "ask thoroughly about what I was", rather than "who I was"; evidently, they saw themselves as something abnormal and unnatural. Gender dysphoria was treated back then primarily by trying to force the affected individual into identifying with their biological sex, and I don't think I need to explain how that could drive someone to suicide. If our Doe was transgender, transitioning probably wasn't an option. Perhaps they thought death was the only way they could truly be at peace. I've no doubt in my mind that they were suffering.
It sounds like Doe might've suffered from dissociation. Having a dissociative disorder myself, I too struggle with empathy, building interpersonal relationships, and feelings of pervasive emptiness, melancholia, and inadequacy. I've struggled with suicidal ideation on and off for the better part of the past decade. Having experienced depersonalisation on a daily basis at one point, I'd say the phrase "parody of a person" is an incredibly accurate descriptor of what it's like to depersonalise. You feel alien. You don't recognise your reflection. You can see your body moving, but you're not the one moving it — it's like someone else has hijacked your autonomy. You're so detached from your emotions and self that you become numb and apathetic towards everything. With derealisation, you feel like you're existing separately to everything else. Like no one else can perceive you. The world is blurry and distorted and utterly lustreless. Truly, you feel like a shell. You feel trapped within yourself. Like you're being forced to exist in a world that has no place for you.
Dissociation is a response to high psychological stress and/or trauma. Makes me wonder about what our Doe went through in life to end up in that tree.
I wouldn't be so quick to dismiss the age estimate as inaccurate based solely on how the note was written. I'm around the Doe's age myself, and I've had people tell me I write like someone at least a half-decade older. I've always been fascinated by words and language, and get the feeling our Doe felt similarly.
Also, regarding Bayard Cousins — I don't think it's him. Bayard had a cross-shaped scar on his finger, which should've been noticed at autopsy given the Doe's body was fresh (found no later than six hours after death). I'm also assuming Doe's age was determined by examining the bone structure, which differs wildly between a 22-year-old and a 16-year-old. But, again, just my two cents. I could be wrong.