But novels are definitely not canon, so they couldn't care less if a novel depicts her as half Romulan.
Ah, but the team at CBS Consumer Products
do care about how Star Trek is represented. They consider the novels (and comics and RPG materials) on a case-by-case basis, and often aim for a consistency of product.
If an author presented a scene requiring Saavik to be full Vulcan, based upon Robin Curtis's scenes in ST III, CBS would evaluate the need of the story versus consistency with previous novels. While one tie-in stated that Saavik had never visited Vulcan until the end of ST III, another showed her being raised there by Sarek and Amanda. There has even been a novel that (vaguely, kinda) suggested that
a second Saavik may have existed ("Unspoken Truth").
A few times, contractual arrangements have meant that Robin Curtis's image
had to be used in DC Comics or on the cover of "The Pandora Principal". Years later, Playmates (and then Diamond) was permitted to do a Kirstie Alley Saavik action figure.
"Not canon" does not ever equate with "not caring".
the rule of "only what was on screen is canon" may not be entirely true.
Canon is whatever the current showrunner (and CBS Consumer Products, whose team critiques every proposal and manuscript) decides it is. Ordinarily, "live action, onscreen, produced by Desilu/Paramount" is canon for the licensed tie-ins, but there
have been exceptions about what else can and has been incorporated in the past, eg. Janeway's background from "Mosaic", but
only until Jeri Taylor left "Voyager".