Prove that "a universe without free will but behaviourally indistinguishable from one that features Compatibilism" is actually a coherent definition/non-empty set.
Compatibilism entails that any universe featuring intelligent agents acting on reasons and capable of understanding and learning from their actions will have free will. So basically, you're claiming that a universe that features such intelligent agents is indistinguishable from a universe that has no such intelligent agents.
> Except I don't need to. Compatibilists haven't proven the existence of free will, so the null hypothesis still holds true, at least for now.
I honestly don't understand what confusion would lead you to make such a statement. Null hypotheses are simply not relevant to this sort of question. What populations are you comparing here exactly?
Secondly, Compatibilism describes exactly what humans do because it's compatible with both deterministic and indeterministic worlds (see point above re:universes with Compatibilism). Compatibilist free will clearly exists in our universe virtually by definition, ie. we are clearly intelligent agents acting on reasons and capable of understanding and learning from our choices.
> Throughout the comments here you seem to continually rely on the fact that professional philosophers strongly agree with the idea of compatabilism
Because most people don't have the requisite knowledge to evaluate possibilities that require profound domain knowledge. Would you be similarly derisive if I referenced the medical consensus when wading through a health debate?