Unlocking a custom 'Next' button after user visits each layer

I'm not on my PC, so I can't look at your file right now. However, I'll offer these suggestions/tips.

• If your POI buttons each have a Visited state, a trigger can change the state of the custom NEXT button to Normal when the state of all the POI buttons = Visited. 
• You don't need a trigger to hide the custom NEXT button when the timeline starts. Just set its Initial State to Hidden.

• This post includes a file that demonstrates various way to control the NEXT button. It's about the Player's NEXT button, but the same instructions would work for a custom button. https://community.articulate.com/discussions/articulate-storyline/tip-controlling-the-next-button-101 

What Judy said. :)

Nice looking interaction. 

I'd simplify your triggers:

• Get rid of the change states to hide object triggers. Instead set the initial state of those objects to hidden. 
• You don't need a trigger to change a state to visited when a user clicks an object since that will automatically happen (that's on the layers). You're using those visited states for conditions. But you don't need to.
• Most of the objects are images and have visited states. There are a few grouped objects that can't have visited states since they're in a group.
• I'd select on of the objects in the group and add a visited state so you can change it to visited like the non-grouped objects.
• Then use a trigger to change the state of the continue button when the states of all the objects are visited.

The state change doesn't work now that I look at your slide because if the object is visited and they hover over it again it is changed to normal.

You should use a T/F variable that tracks when a layer is visited. And then when all of the variables are true you can change the state of the continue button.

However, you don't know which layer they're on so you need to add a trigger to account for each variable change, for example, change the state of the continue button to normal when Var B = True on condition that Variables A, C, D, and E are True.

I'd create a simple check-off system to indicate they visited everything and use that.