I don't really see how any of the extra material from the novels would be helpful here. It seems superfluous at best, and dubious at worst - why would Saavik conspire, and why would the details of Spock's will be an issue?
When Sarek confronts Kirk, it's initially for Spock's katra. The issue of Spock's body does not arise. Sarek probably has no use for that piece of rotting meat - and moreover, he says he has read Kirk's report, so he has reason to believe that Spock's body is now buried in space, or burned to ashes in atmospheric entry.
Sarek only starts yearning for the body after melding with Kirk. In that event, Sarek's conviction about the survival of Spock's soul and Kirk's conviction about the possibility of Genesis reviving Spock's body come together for the first time. It should be no wonder that Sarek at that moment changes his plan, then: he now wants to bring Spock back to life, not merely deposit his katra in some moldy old pot. And Kirk is of course immediately up to speed, too, since mind melds work both ways.
The only thing that needs explaining is Kirk's conviction that the body does survive. And in both the script and the final edit of the movie, the Grissom
has already observed and reported the soft-landing of Spock's coffin, and the fact that it is now empty. So... Does Kirk know what Starfleet knows? This sounds reasonable, but OTOH Kirk isn't jumping with joy when Sarek arrives, so perhaps he doesn't know. Or perhaps his belief is not strong enough that mere empty tomb would convince him.
But all the same, ST2 already had Kirk expressing his belief that Spock might be resurrected: "If Genesis really is life from death" and all that. Combine that with Sarek's knowledge of Vulcan rituals, and everything makes sufficient sense.