Best Practices - Timline and Triggers

Former Storyline 2 and 3 user here, currently trialing 360. From an LMS, I'm delivering an embedded video, several minutes long, streaming from Vimeo. Since embedded videos don't come with the controls and settings of Storyline's native player, I've developed some workarounds.

First, in an attempt to encourage the user to complete the video, I've placed an empty rectangle over vimeo's control bar. The state of this rectangle changes from normal to hidden when the timeline ends, and the timeline is the same length as the video. Not perfect, as it assumes the user starts the video right away (not all browsers support autoplay, and they still pause), but it's something. Once the timeline ends, the user and scroll through and replay the video. Another button changes state from hidden normal when the timeline ends. This opens a layer with key words and a summary.

Second, I've also done the ol' set slide1complete to value true when the timeline ends on the slide. When this happens, they may proceed to the next slide with the next button. 

Unfortunately, after working pretty well, we've started to encounter some issues. For some users, the video (and presumably the timeline) ends, but the rectangle remains and no buttons appear. In this case, they can't advance to the following slides or search the video. Between different browsers, and different machines, this issue has been difficult to consistently recreate. Oddly, however, there are cases where the timeline triggers DO activate some time, perhaps a few minutes, after the video (and, again presumably, the timeline) ends.  

I've uploaded new versions of the course to the LMS. The rectangle now changes states 3 seconds before the timeline ends, and show until end is disabled. All other objects are set to show always. So far, this appears to be working, but I can't be certain. I'd like to know what some of the best practices are that I can apply here. Is there something I'm missing? How does this work sometimes, or most of the time, but not always? What could explain the timeline triggers occuring in such a delayed fashion?

It really doesn't seem like I'm asking all that much of the program- certainly not stretching it to anywhere near its capabilities. Any feedback would be much appreciated.