Good intuition...and thanks for staying the course on this one.
The interactive TOC objects do not have pause points, nor do they have number of attempts because they are set to display for the remainder of the project. I've attached screenshots showing the available properties and AA. They are smartshapes with the Use as Button property checked. Each has a rollover state to change the color of the image. The actions are not conditional. The Open_TOC has 2 actions (Show TOC and Show close button). The Close_TOC action is exactly the opposite.


You don't have a toggle button but two shape buttons? They don't have a pausing point, OK.
I will try to explain what happens. Those shape buttons also have 'Failure', when clicking outside. Of course, you don't have a Failure caption for them, but the event remains. When previewing Project, apparently CP is confused about what to show and ends up not showing anything on Failure. However I got it working for both Preview in Browser and Preview HTML5 in Browser by making sure that the cliick box is higher in the stack than the shape button timed for the rest of the project. I used the Arrange menu, brought Click Box to Front and the project timed shape button to Back. This can be an issue however, you have to be sure that the project timed shape button is never covered up by a slide object. Can you try out that workaround? I cannot remember if this was also necessary for previous versions.