why would Sarek not know Spock's choice? That is a little easier to explain. Regardless of their disagreements, perhaps Sarek would still want Spock to participate in the Vulcan afterlife tradition of having his katra available for future generations of Vulcans to benefit from his experiences. Perhaps Spock didn't want to have that argument.
That was always my interpretation. They weren't talking as father and son again until the end of ST IV.
As for Saavik, it's highly likely, given her upbringing, that she's never been informed about katras.
I read in one novel (I know, it's not canon) that Vulcans perceive their own versions of a God. Perhaps that would also mean that they believe in different afterlifes, and Spock didn't think his being launched into space would affect a passage to the afterlife. Of course, that's just my speculation.
That too. Perhaps the idea with communing only with dead Vulcan minds for eternity wasn't appealing.