Re: If VI had featured Saavik instead of Valeris as the traitor
Either Kirk or McCoy should have been familiar enough with details of Chekov's family relationships to know whether there was a brother, rather than taking Sulu's word for it. Both of them must have reviewed his personal and/or medical files. Accepting Sulu's word as authority seems questionable, as this is the guy who thought himself one of the Three Musketeers, and went loopy over Landru.
Curly: Moe, Larry, the cheese! Moe, Larry, the cheese! (Horses Collars, 1935)