Nostalgia Chick made an excellent point about the first movie. The first 20 minutes or so of the film exist purely for the viewer. It's a set of two short stories in-universe. Both episodes end up neuralizing one of the heroes so only the viewer (and Agent K) knows these events exist.
I'm not sure if there's a trope for that but it's a terrific scriptwriting which is worth mentioning.
No, not in this case (although K neuralyzing agents at the start and then having J neuralyze him at the end would probably be Book Ends of a sort). These are more akin to a Teaser.
Wouldn't the first 20 minutes qualify more as a Cold Open than it does as a Book End. Since it shows us what they do, while the rest of the movie is spent telling us what and why they do it? Getting us into the action early so to speak.