Having Saavik as the traitor would have been a bold choice, and would have made Spock's fury more justified IMHO.
...But what would have been even bolder would have been to introduce this suspicious Valeris character from out of the left field, and then have Chekov
be the traitor!
I mean, Koenig has often been a proponent of doing "spooky" and "dark" Trek, and likes to play dark characters (and was particularly delighted to get the Bester gig). He might well have loved to get written as a conspirator and traitor who almost gets to kill Kirk.