Right now, there is nothing that prohibits a deadmind (oops, I meant "a sterling example of mankind untainted by the warp") learning Psyniscience and detecting psychic powers with that - skills have no prerequisites and there are no advancement tables to say what a character can or can't learn. Is this intentional?

I think it is hidden like in Black Crusade under the Trait Psyker, which only the Sanctioned Psykers gets. In this trait is states that only with the Psyker Trait someone can learn Psyniscience, Psi Rating and Psychic Power Talents.

On page 111 in the description for the Psyker trait, it states that a character with the trait is able to purchase Psyniscience, Psy Rating, and Psychic Powers. There definitely needs to be a clause in the skill chapter stating that only Psyker characters may purchase Psyniscience, though.