I have written screenplays, so my author-brain reflexively tries to figure out how I would plan any scenario for maximum impact.
I actually hypothesized that Jason - the son - was going to get switched during the movie and that was the point of the mask, to hide the scarring until it was revealed. I thought the attack on the little girl as shown in the trailer was going to be the initial prologue incident and that whatever lab/hospital shown was specifically trying to capture children as fodder for tests and for genetic material to gear up an underground cloning business whose intention was to fix and replace "damaged" and dead loved ones for the wealthy - similar to how some parents will replace children's small pets like rabbits secretly without informing them. That was how I interpreted the rabbits in the trailer: low-attachment, easily-replaceable pets. I thought all the people in that hallway were the leftover lobotomies like in Get Out.
Or the entire family except Adelaide was going to get switched out over the course of the movie one by one but continue along and her terrifying scream at the end of the trailer was one of betrayal - the realization in the finale that they ended up saving the clone, changeling-style, and that would have turned out to be actual Jason in the car-burning fire. And that was the purpose of the jumpsuits and the scissors: quick deceptive clothing swap.
Then when I saw it and the movie cut away and held back the reach through the mirror shown in the trailer, I knew they were saving it for a reason, and it all made sense why Red's speech was so broken and why the others didn't talk.
Lupita Nyong'o is AMAZING though (as is the entire cast) and it was all worth it, including the shivery implications of that very last moment. I'm considering going back to see an evening showing to experience it with a more responsive and hopefully clueless audience!
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