This is a highly contentious speculation

Hmm, I see I wasn't clear there at all. That all levels can be deduced from one level is just what Deutsch himself isn't granting—he argues against it! Rather, it (or something like it) is the explanation I usually see people give for why they label the bottom of the blue line as more "fundamental"; my intention was to point out with the next sentence that the complete-deducibility hypothesis doesn't make the blue line directional even if true, because it would allow travel along the line in both directions. I definitely need to rewrite that part. (All that being said, I do think that advances in physics letting people design new chemical processes, and that sort of thing, are strong evidence that far more is possible; I find the hypothesis more plausible than you and Deutsch do.)

together forming "explanations"

It's true the directions of my green arrows are debatable; the reason I went with upward was because the simplest way I could think of to formulate what was happening was just "X explains Y"—"many compounds explain synapse." I agree that in a more zoomed-in view each green arrow would imply a complex up-and-down motion of analysis and synthesis.

it seems to connect towers of abstractions together somehow

Violet lines of causation connect blue lines of territory to each other, not green towers of map. Green towers are connected by red threads, which are (causal) explanation, not causation. I thought this was clear by analogy with the first two diagrams, but you're not the only one, so it looks like I should make it more explicit.

gratuitous use of pathos and evocative imagery

It's a fair cop! But I like it that way. ^_^