There's no epiphenomenal type of stuff in QM.

Why is that important? The obervable difference between epiphenomenal type of stuff (= never interacts) and quasi-epihenonemal causality (=rarely interacts) isn't necessarily an observable differnce. If branches of the multiverse only interact once every billion years, then multiversal theory predicts effectively nothing about expected future experience. (I don't personally have a problem with saying mutliversal epiphenomenaism is better than substance epiphenomenalism, but that is because I am not commited to the prediction of exepcted observations [warmed-over LP] over and above Best Explanation and even good old fashioned metaphysics).

An why bring up substance anyway? Contemporary epiphenomenalism doens't focus on substance, it focusses it on properties (Jackson, at one time, Chalmers, maybe) or laws (Davidson).

There's just a causal type of stuff, some of which got far enough away that under the standard and observed rules we can't see it anymore. It's no more epiphenomenal than a photon transmitted into space or a ship that went over the horizon.

OK. So, you are willing to countenance theories that don't pay their way in expected observations so long as they pay their way in other ways...

Deducing an epiphenomenal type of stuff would be more difficult, and AFAICT would basically have to rely on there being structure in the observed laws and types of your world's physics. For example, let's say you're in the seventh layer of a universe with at least seven causal layers. The first layer has seven laws connecting it to the layer below, the second layer has six laws connecting it to a layer below, and then you're in the seventh layer, connected by two laws to the layer above. You might suspect that there's an eighth layer below you, and that the single remaining law is the one required to match the pattern of the seven layers you know about.

That was cast pretty much entirely in terms of laws, although the contemporary arguements lean much more heavily on types--on what things are, on what their natures are.

A typical argument would go:

*1. Physical brain states (or at least the physical properties of brain states) are sufficient to explain observable behaviour.

*2. Consciousness (or at least qualia) cannot be directly identified with the physcial properties of brain states... they are different types of thing, their natures are differnt...

*3. Therefore, qualia are not needed to generate behaviour...they are extraneous and idle.

I don't see how causal diagrams help. If you feel that conscious states can be identified with brain states, you would draw a causal diagram with nodes that are psychophyscial, and if you you feel that they can't, you would draw a diagram with a physcial network and a consicous network in parallel. I don't see how causal diagrams tell you how to identify and classify nodes -- they ratherf assume that that has already been sorted out, somehow.