Vorlons may be telepathic, but they (or at least Kosh) may also be sufficiently respectful of their allies not to scan them uninvited on first meeting.
Lyta says
right in the dialogue that she will be reliving Kosh's experience as herself, and therefore she'll see
her body, not his. (Yes, creaky, but that's the explanation given in the film itself, so I see no reason to quibble with it.) And in the picture it is woman's hand. The glow effect was supposed to make this clearer.
Kosh short-circuited the entire welcoming procedure by coming aboard early and unannounced. He may not have expected to be met by
anyone right inside the docking bay, much less Sinclair. The humans were all rushing to reach the bay in time to greet him and barely made it. Nor is it necessarily the case that the Minbari would have told everything to the Vorlons. ("The Minbari never tell anyone the whole truth") They may have withheld Sinclair's identity, or they may have not revealed that in Sinclair they had identified the
particular soul of Valen as one of the captured Humans. Hence Kosh's
surprise at recognizing Sinclair as Valen (and there's no reason to suppose that the Vorlons could not see past the relatively minor physical differences between Sinclair and Valen.)
Although the line was added only for the TNT re-edit, JMS has said that he
wrote it in an early draft and cut it before shooting because it was a potential spoiler. And I'm pretty sure he indicated that it was supposed to be a moment of recognition - indicating that Kosh was not
expecting to see Valen in whoever greeted him.
I think the "handshake" was a spontaneous gesture, based on a tradition that Valen may have introduced the Vorlons to, and which the assassin anticipated. (Or the assassin had some more direct means of peircing Kosh's encounter suit. Don't forget, they are primarily worn for concealment, not protection, and Kosh's suit was damaged more than once. Since the Shadows probably indirectly supplied the poison, there's a good chance they also supplied a way to open Kosh's suit - dumb luck he saved them the trouble.)
Regards,
Joe