Hide Show next slide button based on visited states

I see your first chevron is part of a group. Please do yourself a favor and ungroup it.

There is an old saying in the theater: "Anyone who puts kids or animals on the stage deserves what happens to them." That means that no matter how well-behaved, or well-trained you think they are, at some time they are going to revert to their true nature, and you can only hope it doesn't happen during a performance.  The SL correlation is: "Anyone who uses groups deserves what happens to them." That means that no matter how well-behaved or how well-trained you hope they are, groups don't play nicely with anything, and especially not states, clicking on, and triggers. Sooner or later, you are likely to have problems with them.

I used a group in SL once, and it worked the way I wanted. I think it was in 19...  No, wait. I'm thinking of another program, Maybe it was ...  Well,you get the point; groups don't work in SL. Pretty much you can use groups, or you can have triggers and states that work, but not both. I would counsel you to avoid them if at all possible, and get rid of any of them that aren't absolutely, absolutely necessary. If you absolutely must have 2 or more objects that stay together, and act as one object (there are a lot fewer of those than most people think), then do it in a way that won't cause you heartache later. Create them all separately, select the next one up, copy it, edit the state of the bottom one and paste the upper one onto the bottom's Normal state. (repeat if necessary until you reach the top). That way, they become truly one object - all the benefits of groups, and none of the potential problems.

Also, just as a design consideration, I open the slide, and I see things that look like arrows pointing to ovals, with labels that make me want to see that information. So I click on the oval that the arrow points to, and nothing happens. I'm frustrated. Why are the ovals there? If you have to have them  so the check marks have a home, I would place them to the left of the chevrons.

As to your problem with the back button, blame this trigger:

It can fire only at the moment the timeline starts, and the chevrons can't be in Visited state yet.

I would use this instead:

That guarantees that the learner not only visits each layer, but stays on it for a set period of time. More importantly, it allows the layers to be visited in any order. One caveat, if you skip ahead in the video, the chevron isn't changed until the end of the layer timeline.