On the topic of persona alchemy, whose event horizon we seem to be pretty much perpetually struggling to escape from, rereading this little tidbit about Caliborn’s transformation from Doc Scratch to Lord English reminded me of something out of the alchemy of the ancient Greeks (the Platonic philosophers in particular) and the Islamic thinkers; the idea that all substances contained various measures of certain fundamental “natures” - hotness, coldness, wetness and dryness - some external and some internal; and that therefore a substance could be transmuted into another substance by rearranging these natures, or by turning the external natures internal and vice versa.
Ergo, just like how in classical alchemy we can transmute lead (cold and dry externally, hot and moist internally) into gold (hot and moist externally, cold and dry internally) by switching the external with the internal, we can transmute Doc Scratch (cueball externally, puppet internally) into Lord English (puppet externally, cueball internally) by doing the exact same thing.
And it seems to me that this scope can very easily be widened to encompass Homestuck’s library of alternate selves as a whole. In one timeline, Dirk Strider’s manipulative tendencies might be exterior while his compassionate tendencies are interior (Equius externally, someone like Nepeta internally?), while in another timeline, the latter comes to the fore and the former recedes.
Which to me implies that the true meaning of the Ultimate Alchemy may actually be something to do with the Ultimate Self… but you might have somehow guessed that already.