Ben,
I haven't read beyond this point in the thread but it's bedtime. I'll probably regret this but I'll throw this out now and read more tomorrow.
You said about the concept of an afterlife, "In the way that it has absolutely NO effect on any natural thing. In other words, it's not measurable in any way, at least as far as I can see. It seems really that simple--definitional. Maybe one step, a simple syllogism."
I just want to point out that believers in an afterlife often believe that it has effects and will point to things just as disasters or good things as being examples of those effects. So for believers the supernatural is an explanation of the manifestation of desired as well as undesired events, also low probability events.
Many of them find the scientific explanations difficult even impossible to comprehend. I will say at this point that the supernatural exists as a primitive explanatory mechanism that is often more satisfying to the way many human brains function than is rational scientific theories.
Best of luck on educating believers in the supernatural! I don't envy you your mission.
lfen