star trek - How much of the plot of The Search for Spock was determined when The Wrath of Khan was written?

Not too much was apparently planned throughout most of the filming, since this article mentions:

Paramount also really wanted Nicholas Meyer to return as director for Star Trek III, but after he read the script Meyer passed. His reason being is that he felt Spock's death should have been final, and to resurrect him so soon was underselling the franchise.

And this interview with Nicholas Meyer quotes him saying that both the shot of Spock saying "remember" to McCoy and the coffin on the Genesis planet were done against his wishes, after the movie had already been shot:

When Paramount saw the movie, when Leonard saw the movie, and everybody said, “Well, gee. Gee. Maybe killing him isn’t such a good idea.” And at that point, we got into the whole thing about “Remember” and showing his coffin on the planet, and stuff like that. All of which at the time I furiously objected to. I just thought this was so unfair to an audience of people who really care about this sh-t, and then saying, “You know, oh, it was just a dry hustle.” No, I didn’t think that was right. And in retrospect, you know, maybe I was wrong about that. At the time, I just thought that my vision of the thing was being insensitively overruled. But that’s when they made that insert, about “Remember” and put him on the planet in his torpedo.

So, this stuff would not have been in the shooting script, since Nicholas Meyer wrote that himself based on drafts by other writers (see this excerpt from Meyer's memoir).

Additionally, p. 249-250 of this biography of DeForest Kelley says that the revised ending was written by Harve Bennett, and that it included the dialogue about "possibilities" on the bridge and Spock's closing voice-over:

The first preview audiences agreed: the shock and grief were too much. Bennett set about to modify the ending. Meyer objected but did not stand in the way of the new material. The rewrite included an end scene on the bridge ("There are always possibilities"), a sequence shot by ILM in Golden Gate Park, San Francisco, revealing Spock's torpedo casket intact on the planet, and the use of Nimoy's voice in the closing Star Trek credo ("Space, the final frontier").

This site has scripts for TNG and DS9 as well as the movies, including the script for Wrath of Khan, and I presume it's the shooting script since it has most of the movie we know but doesn't have the "Remember" scene. It does feature a description of Spock's coffin being shot off towards the Genesis planet:

Spock's remains seen on their way to the new world.
The capsule grows too small to SEE. The bagpipes STOP.

But just because Meyer included this scene doesn't necessarily mean he was thinking in terms of the planet resurrecting Spock, it may have been more like a poetic juxtaposition of the end of Spock's life and the new life represented by the planet (and Kirk's line to McCoy that he feels "young" is also in this script, which would fit this sort of contrast).