Steal Across the Sky: huh? (!! spoilers !!)

I don't understand why the observations of the Witnesses in Steal Across the Sky support the major premise.

(!! spoilers !!)

The Witnesses are taken by the Atoners to planets populated by humans removed from Earth 10,000 years ago. On the control planets, these humans are unmodified (unlike back on Earth), which means that they still have the gene that lets them communicate with the dead. Hence they have direct evidence that there is indeed "life after death". So far, so good.

However, in the piece we see in detail, we learn that live humans are on the "first path", with the recently dead humans (the ones the live humans can detect, and converse with) are on the "second path". But they don't stay around on this second path for long (a few weeks, a few months at most, it seems), before they go off on the "third path". It is this "third path" that appears to be the "real" long-term life after death, yet there is no more evidence it exists than we (unmodified humans back on Earth) have for the second path. Those on the second path don't seem to be able to communicate with those that have gone on to the third path, any more than modified humans can communicate with those on the second path.

So, the excised gene lets people see that when you "die", you hang around for a bit before disappearing of somewhere mysterious. But evidence for a couple of months hanging around chatting to the folks back home is one thing. It certainly isn't anything like evidence for "life after death" as assumed by all the characters in the book. Yet the whole premise appears to be that it is sufficient evidence. What gives? Did I miss the chapter where this is explained?