Here’s a better explanation of my initial issue, along with screenshots. I decided to add it because I’m still having the same issue on this last movement.
1. Here’s the initial image before changing anything. I have one track of video (speaker), and two pictures that will composited on top. I want to keyframe a movement of the speaker’s video getting smaller, along with moving to the left to make room for the images. To do so, I set my playhead at the first frame of the split clip at 21:41:00, and set my first keyframe.

2. I move ahead to 21:42:04 to set the end of the movement. I use the numbers method by double clicking on the X value positioning under transform in the inspector. I type in the numbers and hit enter. The next keyframes are set.

3. Everything’s fine… As the pictures transition out, I want the video of the speaker to scale back to 100%, and the position to go back to 0,0. To make sure that the image dosen’t start moving until the beginning of the still image out transition, I move the playhead to 22:41:08 (the beginning of the first out transition), and set new keyframes by clicking on the keyframe “diamonds” next to position and scale under transform in the inspector
BUT WAIT–what’s that?? All of a sudden, a rogue keyframe comes in that has the image go completely OFF SCREEN, although I didn’t set it to do that.

Here’s another picture of viewer window zoomed out to show the rogue movement.

That’s my issue. I’ve gotten over this by adding cuts between the keyframes and just turning it off–but I shouldn’t have to do that, should I?