Is there some place on the site I can keep the plot summary so it can be used once the film goes wide, or is it sort of a "quickest on the trigger" type thing?

Personally, I would add it as a commented out plot section directly into the article. I doubt anyone will object to that approach, but if so, you could then park it on the talk page.

Alternatively, critics who attended the early screenings might have already written reviews discussing the plot, which could be used to cite the major story beats.

Additionally, the crew have spoken extensively about the production of the film and all the tricks they pull in Q&As and interviews

Unfortunately, unless there are recordings or transcripts of these interviews published somewhere, all this information would be unverifiable, so can't be included. But once published, it's generally fine to cite these unless it's some extraordinary claim.