Elroy, I think your solution is fine.
- Yes, it's possible to use a variable in the trigger conditions, but then you'd need extra programming to control the variable. So that solution is great when you need to track what happened across multiple slides.
- Since this is all on one slide with objects that have a Visited state, I think it makes the most sense to use states in the conditions.
The only suggestion I have is this: change the NEXT button to Disabled. That keeps it in its place and also keeps the PREV button in its place. (When NEXT is Hidden, the PREV button moves into that area. And someone might click it thinking they're clicking NEXT--and be very confused when they're at the previous slide.)