Thanks very much for all the details. If the only thing you were looking to do was to immediately cut to that triple view for a few seconds, then trimming the main clip like you did and moving the screen position of just that clip down was the simplest way to go. No keyframes needed at all.
If you wanted to use keyframes, you could have left the clip intact and simply placed a Position keyframe at around 1:59:00, followed the very next frame (1:59:01) with a new Position keyframe with a Y value of something like 0.9.
You'd then place a 3rd Position keyframe (with Y still at 0.9) at the last frame where you wanted to main clip to be in the lowered position, followed one frame later by the final Position Y keyframe with the value set to 0.5. The clip would instantly drop and return at the desired points in the video.
Take a look at the sample project where I used just 4 keyframes to move the main clip just like in your video.
I also used 4 keyframes in the Mask Designer to remove the top part of the lowered clip so it didn't cut off the bottom area of the two upper clips. Open up the Step Down Keyframe project (from the same link in my earlier post) and experiment to see how things work.