Kirk simply broke Vulcan tradition by a) shooting Spock's dead body into space and b) not returning his katra. Sarek wanted to bury Spock at home and store his katra somewhere.
They never intended to put Spock's katra back into his body.
That's a very good interpretation of the movie and it would make a whole lot more sense if they said so explicitely...
...unfortunately when Kirk talks to the admiral about going back, he seems already under the impression that he's going to bring Spock back. What convinced him was the video footage from the rental store... er... surveillance camera. It seems your interpretation, though preferable, is incorrect.