I'm also pretty confident that the writers and producers knew that the plot of the movie was contrived and full of holes, but decided to go with it anyway on a cost/benefit type of analysis. Chiefly, they must have decided that having Nimoy's Spock around was worth the time-travel contortions required to bring him into the story.
Ironically, one motivation for this was probably to please longtime fans, some of whom seem to be among those most displeased with the contorted plot. Whatever. They thought it was worth it. That's what we have.
The box office receipts would suggest they were right.
Yeah, none of them are dummies, I'm sure they probably did sit down and say, "well some people won't like this at all." But it those people represent less than 10% of the core fanbase, and you bring in more than enough new fans to compensate, well, that's business.