I never saw Wrath of Khan theatrically, so I'm unaware, but when Spock died (which was a legitimately awesome moment) was it meant to be for good or did they always have the Genesis resurrection thing up their sleeves? It kind of takes away some of the impact from the otherwise awesome Wrath of Khan knowing that used some hokey plot device to bring him back from the dead.
Anyone's guess, but being that III came out just 2 years later, I find it difficult to believe anyone behind the cameras thought he was going to stay dead.
That said, despite a decent box office, the original movie was considered a disappointment, so maybe the brains behind the movies figured it could actually end up being permanent.
My take, without knowing the background: They thought this would be the last Star Trek movie. If they had already begun developing a story for his comeback during the course of making II, I doubt 3 would have ended up as shitty as it did.
So. . .little from column A, little from column B. It's all a rich tapestry.