That last comment from Hussie on p.387 of Book 6 also uses the term “Persona Alchemy”, which he’s used before but I never really picked up on; it sort of explicitly ties it back to the concept of Platonic idealism used within Homestuck’s “actual” alchemy, and therefore to the sprites, which operate on the very same philosophy. It just ties everything together neatly.

And then there’s this comment on p.382, which I can’t really think of a way to incorporate into any other post, but definitely feels like it needs to be included somewhere for posterity:
I remember Aradia whispering to her past self being a cagey way of implying that she was the one responsible for her own ghostly communications […] it’s kind of an interesting idea that this is demonstrating the mechanism which allows for her to receive messages from dead people. It would mean that ghosts are actually communicating with their memory of Aradia. And ghosts in their bubbles whispering to the memory of a psychically gifted person may serve as a conduit for actual messages being delivered to the real person. It opens up some deep questions. Is your memory of someone linked to the truth of that person’s existence in some way? The topic isn’t explored much beyond this one cagey whisper. But we do know that in Homestuck, ideas of people and things have a lot of power. Would it be unreasonable to think influencing the idea of a person might hold some sway over the physical instance of that person?
It’s an interesting angle to see inhabitants of the dream bubbles not just as the ghosts of dead timelines, but as memories of those timelines; literally the “versions” of people that live in their own or other people’s heads, yet another kind of reflection of the “true” Platonic idea of someone. Which I suppose is also literally the domain of the Mind aspect;