How could the Minbari think that Sinclair, a
Human, was a descendent of Valen's? Nobody at the time said anything about DNA or anybody being a descendent of anybody else. They already had a belief that their "greater souls" were not being reincarnated as Minbari. They saw the triluminary glow and
immediately summoned Delenn. They did this because they thought he had a
Minbari soul - not Valen's.
Some thought that Valen's soul might have been reborn in Sinclair in whole or in part, but others rejected that idea from the outset and merely thought it was a Minbari soul. But that's the point: They
only speak of souls and they
only speak in terms of the Triluminary. Delenn's experience further underscores this: The Triluminary rarely glows for
anyone. Those for whom it does are presumed to have "great souls". So Sinclair (and the other humans) not only had Minbari souls (as far as the Minbari could tell), they had
great Minbari souls.
IIRC Dukhat never uses the term "Child of Valen" with reference to Delenn, never really explains what the glow of the triluminary means. He never gets around to that before he's killed. We only start hearing about Children of Valen in "Atonement" after Sinclair's identity is discovered. The whole notion that a glowing triluminary has something to do with DNA doesn't arrise until then. It makes no sense to read it retroactively back into a time when the Minbari thought it indicated a great soul. So there's no reference to any other meaning at the time of the Minbari War, and absolutely no reference to anything else being used to test Sinclair and the others. Why assume that some event we've never even heard of was happening? If we can do that I can come up with a whole alternate story line in which Sheridan himself was really a Vorlon but all the stuff that supports this happens "off-screen" and is never discussed. It is one thing for
In the Beginning not to show the testing of the other Humans because of time constraints. It also doesn't show the attack of the soul hunters for the same reason. But we already
know about both these things from earlier episodes, so we
can assume they take place off-screen. We have no such justification for believing that other testing methods were being used or that anybody said that Sinclair must be descended from Valen "off screen". (BTW, how could Sinclair have been descended from Valen given that the first contact with Earth only just happened, and that 1,000 years ago interspecies gene-splicing must have been even harder than it is today? We already know that interspecies breeding is impossible in the B5 universe. And no, I haven't forgotten G'Kar and Lyta in the pilot. I just don't believe that a "direct mating" could have produced a child - and I don't think G'Kar believed it either. We all know how fond he was of Human females. He just figured Lyta wouldn't know any better and thought he could get her into bed with that line. Can't really blame him, myself.
)
Did Lennier say that the other Humans were tested with the triluminary alone? Not that I recall. But why
would he, since there was never an action, an image or a line of dialogue the suggested that anything other than the triluminary's glow made the Minbari think that Sinclair had a Minbari soul?
Did Delenn say the Minbari were wrong about the soul migration? Not directly, as I recall. What she said was that they
thought the triluminary glowed because Sinclair had a Minbari soul, perhaps Valen's, but that they now knew that Sinclair and Valen were the same person - and that it was reacting to Valen's Human DNA, not his soul. That's not an explicit statement about Minbari belief in the transmigration of souls, but given that all indications are that prior to "WWE" everyone, including Delenn, believed that the glow meant "soul", and that she has now accepted that it was detecting DNA, that pretty much means the soul theory had to be wrong.
To use a crude example, if an archaeologist announces that he has firm evidence that Jesus never existed, but that he was created as a literary figure by a splinter movement of Judaism, you really don't need him to add an explicit statement saying, "Oh, by the way, this means that Christianity is false." That's would be kind of a given.
Regards,
Joe