Vejur said:
OMG Spock died

yank, yank, don't you worry he will do Jesus Christ trick in TSFS
Not at all. The way the script for ST III was conceived and constructed, there was never a certainty that going "In search of... Spock" (ie. parodying Nimoy's documentary series) was going to find Spock alive, or that Nimoy would be playing him.
The treatment "Return to Genesis" had the various characters looking into mirrors and seeing a vampiric Spock creature staring back at them, warning them, during their ongoing mission to prevent Romulans from stealing the secret of Genesis.
This creature may or may not have been played by Nimoy - but signing Nimoy as director was very clever, since now anyone watching Nimoy drive through the Paramount gates during production had no idea if they'd just seen the director or one of the featured players. And great free publicity to have "Spock" direct the movie sequel to the film in which he'd been killed off. The media loved it!
Saavik and David had been added to ST II with the idea that new, younger characters would be needed if/when ST returned to TV as a series of telemovies, and as the TOS cast continued to age. Especially since they'd already lost Will Decker and Ilia (both were intended to be ongoing characters for "ST Phase II", and both originally survived "In Thy Image"). All the "Phase II" scripts had vacant spots for a young male lead, a smart/mysterious science officer, and an attractive, young leading lady.
The strong rumour, as ST III was being written, was that
Saavik would somehow be channeling the memories of the lost Spock during this new adventure. Kirstie Alley had already been groomed and promoted as a Nimoy replacement in the cast, but suddenly with ST III's Director Nimoy, he wanted to go in a very different direction, and he essentially reconceptualized the character by casting (and directing) Robin Curtis.
And, as the script shows, the timing of the regenerated Spock's growth spurts (ie. tied to the seismic activity of the planet),
any younger actor could have been playing Spock when he got beamed up. So Nimoy's safety net with the movie meant that, if he wasn't wanting to actually play Spock again, they could have beamed up Joe W Davis as the planet blew up and fandom would be celebrating a new Spock for the next ST production! (And young enough to play all the lines assigned to Xon in the "Phase II" scripts.)