What would Spock's katra have to do with a potential Saavik/David child? It was McCoy who had Spock's katra!
I am assuming, of course, that in this other would-be third film there would be no Spock and Bones still couldn't keep it without mental damage or whatever, they'd still need to find an alternative vessel. Insert silly sci-fi plot device here.
Not necessarily. Remember that when Star Trek II
was filmed, no one knew what "Remember" meant. We look at it now, from the perspective of Star Trek III
and beyond, and we say, "Oh, it's so obvious. Spock gave McCoy his katra
," but, at the time, it may have just been Spock's way of saying, "Remember me" (which is what Meyer intended).