Good essay! And, it's good to hear from you again.
"..the criminally simple question "Who am I?" remains unplumbed and forever enshrouded in mystery."
I'm more optimistic than that. I think there has been some progress toward answering the question, and I think that it might be possible to lift the shroud and reveal the mystery some day.
"...we can at the very least define with great accuracy what we aren't."
I agree, and I think this represents some degree of progress. I was impressed, long ago, by Julian Jaynes' exposition of what consciousness is not in his "Origin of Consciousness in the Breakdown of the Bicameral Mind". I don't think he did much to reveal what consciousness is, or how it originated, but his list of what it is not got me started on a long train of thought about what it might be.
"...it truly is remarkable how quickly most people's conceptions of the soul crumble to ash when the right questions are asked."
In my opinion, David Chalmers has done the best job of showing the fallacy of reductionism in explaining consciousness. (See "The Conscious Mind: In Search of a Fundamental Theory" and Chalmers' subsequent defenses of his work.)
"What sense does it make, then, to treat oneself as one unchanging soul, one unchanging essence, one unchanging consciousness...?"
To me, it does not make sense.
"Instinctively, culturally, egotistically and naturally we all assume our identities to be unique and unchanging."
You might be right about that. But I, for one, have gotten past the instinct, the culture, the ego, and the natural inclinations and have come to the logical conclusion that our "identities" are not unique and unchanging. Thus it makes no sense to refer to them ("our identities") in the plural, nor to conclude that it (the single identity) is unchanging. In my view, there is but a single identity, or "I", which is perpetually changing.
"Thus, it would seem that any logically consistent model of consciousness must do away with the notion of an inviolate, unchanging "I.""
"I" couldn't agree more.