Triggers for Hotspots

I suggest you don't use hovering to reveal layers. Someone navigating with a keyboard or viewing the course on a touch-screen device can't hover. To reveal required content, it's better to require the user to select an object, which can be done via clicking, tapping, or keyboard entry.

I also suggest using transparent shapes instead of hotspots. Here's more info: PRIMER: Hotspots vs. Transparent Shapes | Articulate - Community 

With shapes, you could take advantage of the built-in functionality of the Visited state. Show your custom next button when all of the shapes are Visited. 

Otherwise, you'll need to track completion of the interaction with variables.

This post is about controlling the built-in Next button. However, the same programming works for showing a custom button: TIP: Controlling the NEXT Button 101 | Articulate - Community 

P.S. For future reference, this Exchange Best Practice section is for general questions about eLearning. You’re asking how to do something in Storyline. This type of question is a better fit in the Discuss Articulate Products section of the website. That's where those of us who regularly answer questions about how to use the software spend more of our time. And that's where more folks go to search for answers.