I will always believe that Nomoy hated the fact that Saavik was a highly hyped and popular charactor and would interfere with his limelight in future Trek movies.
A very strong rumour about "how could Spock return?" after ST's premiere was that Spock implanted "Remember" into McCoy to tell him that Spock's life essence had been transferred to Saavik. It was no secret that Kirstie Alley had been groomed to be the replacement Vulcan.
And yes, I think the main reason we never saw that "half-Romulan" quote was because Nimoy himself had had the opportunity to retool the character for ST III.
Vonda McIntyre produced a minor miracle by making her novelizations of ST II and ST III seem so seamless!
Mind you, I have no problem believing that, as a result of Spock's death, Saavik reacted by going into "full Vulcan mode", tempering her own behaviour as she had not been able to do earlier.