I think I completely misunderstood what you've talking about. The first video shows the zoom bouncing back to the original position at the end. I thought that's what you were talking about.
What's not working in this video? At the very end of the video I see it zoom in to the image. Are you talking about the snake curve? That's been in FCP since 1999. Scale is a logarithmic motion, position is linear. The two have different behaviors and are calculated differently, and they will never match exactly to make a straight motion in FCP. You can use the bezier handles to smooth this out a little but it will always be there the way the two are calculated. To do a smooth motion, without easing, you can animate the crop/crop function. Use the default fit spatial conform.