I recognize that my map is a different portion of the territory, that portion of the territory which resides inside my head and which is merely correlated to the actual territory outside of my head.

But the actual-territory is not (or at least, need not be) causally influenced by the territory inside your head that's implementing the map.

But tsn't that the opposite of what you and Tegmark IV are saying: namely that the maps we call "equations" represent something ontologically basic about the whole of the territory?

I can't speak for Tegmark, of course, but what I'm saying is that "equations" are the territory, and the stuff that looks to us like rocks and trees and people and the Moon is just a map.

I'd be careful what I'd call "nonsensical" when your argument instead must lead to the conclusion that things must exist even when they don't exist...

On the contrary, the conclusion is that things must exist even when they don't "exist" - where that quotation refers to some silly little savanna-concept we have in our brains, about rocks and trees and people and the Moon. Which don't exist.

Epiphenomenalism is usually referring to something conceptual or cognitive (e.g qualia or consciousness or subjective experience) that doesn't have an influence on physical states.

You're instead talking about physical existence not having an influence on conceptual entities (namely the mathematical equations).

That's because (in my model) the conceptual entities are the bedrock of the hierarchy, and physical existence is strongly analogous in this model to qualia in a physical-realist model. After all, "{equations}" and "rocks following {equations}" both give the same result for "value of X at time T", so the existence of rocks is epiphenomenal to the equations.

once you have the physical reality of a mouth and a keyboard (which happen to require mass-energy) to be able to say to the equations, you can just say "equations"

But a simulated me, existing only as information represented by electrons in a computer, could say "equations" just as loudly. So why couldn't a purely informational me, existing as unrepresented information, say "equations" too? Physical reality is a burdensome detail which doesn't add any explanatory power to your model; the claim that information needs to be represented in order for conscious entities contained within that information to exist seems to me to have no evidence backing it up, nor indeed to be capable of having such evidence, and therefore Occam demands that we frame our model in such a way as to make that claim inexpressible. It's rather like moving from configuration space to relative configuration space; unmeasurable claims become unreal.

But it still doesn't conclusively explain how the territory came to be.

It doesn't need to "come to be"; 'time' and 'causality' are parochial notions, concepts we can use to model things within our universe. Expecting the multiverse to obey them seems to me to be a Mind Projection Fallacy. A block universe just is.

Thanks for the insightful critique, by the way - it's helping me to understand the arguments better and see weak points that I wouldn't have noticed myself. I'm still not sure whether my theory is circular, nor whether I should care if it is.