Sarah, I am so inspired by this! I adapted this to create something for my practicum in my graduate program. I did run into a few issues that I was able to troubleshoot. I thought others might be interested in case they are experiencing the same:
I was able to bypass the boundaries on the left and right by clicking the left and right arrow keys on the keyboard rather than on the disable left and right buttons. This allowed me to go past the boundaries. I fixed this by adding an if-statement to the key press triggers. It says "If the state of Left Button = Normal" so the left keyboard key only works if the corresponding left arrow button is normal and not disabled. I did the same on the right.
I also had trouble with the left and right stops being invisible. When those boxes were invisible it did not recognize that the object was intersecting and therefore did not disable the navigation buttons. I wound up color-matching the box to the ground/side-walk so it was still technically not-visible to the learner, but had an assigned color. That did the trick, and now it recognizes the intersection of the objects.