But for me it's hard to see why "reality-fluid" (the name I give your "measure", to remind myself that I don't understand it at all) should dovetail so neatly with the information needed to locate events in universes or universes in Level IV.

Well, why would it be easier to locate some events or universes than others, unless they have more reality-fluid?

It's clear why an epistemic prior is phrased this way - but why should reality-fluid behave likewise? Shades of either Mind Projection Fallacy or a very strange and very convenient coincidence.

Why is it possible to describe one mathematical structure more concisely than another, or to specify one computation using less bits than another? Is that just a property of the mind that's thinking about these structures and computations, or is it actually a property of Reality? The latter seems more likely to me, given results in algorithmic information theory. (I don't know if similar theorems has been or can be proven about set theory, that the shortest description lengths in different formalizations can't be too far apart, but it seems plausible.)

Also, recall that in UDT, there is no epistemic prior. So, the only way to get an effect similar to EDT/CDT w/ universal prior, is with a weighting scheme over universes/events like I described.