Video timeline tie to seek bar

Your issue is not really if the video timeline can be tied to the slide timeline; that's easy. The real issue is trying to lock and unlock the video by thirds.

If all you want is a video timeline tied to the slide (Storyline) timeline, do this:

take the seekbar off the video and start the video when the slide starts. That puts both the video and slide on the same timeline, which can be controlled by a seekbar on the slide.

Although it occurs to me that if you use only a video seekbar, you might be able to place three invisible shapes over the corresponding parts of it. If those shapes are filled with color, and made 99% transparent, it might work. When the seekbar is moved, moving over the shape might break the connection between the cursor and the seekbar, preventing moving it into a locked third.

Edit - A second thought makes me realize that while that would prevent moving ahead, it might prevent moving back. I'll have to test it.