The test for an unbiased definition of a philosophical position is (surely) that it is equally acceptable to critics and defenders of the position.

Disagree. That test is biased in favor of positions held by people who are better at strategically taking offense to definitions that are not biased in their favor.

I am trying to capture the particular approach and terminology of this site (from initial impressions) in an unbiased way.

If I understand it correctly, reductionists on this site believe that, for the purposes of causal explanation, any “territory” in the sense of physical reality is best characterised as corresponding only to the lowest hierarchical level of our best map of it, higher levels of organisation existing only in the map. Is that right?

You're strawmanning; we're not idiots. Saying that the universe directly computes quarks is the conjecture that our current map is fine enough that it contains objects referenced by the simple principles that give rise to everything else. I agree with Psy-Kosh that it was a mistake to mix that conjecture with an explanation of reductionism.