After clicking a master page item, go to a layer rather than go back to the base layer?

Can't look at your project, but you can try these:

1. Set master slide layers to not hide other layers so you don't lose the next button
2. Mask or set slide layers to disable base slide objects at some point so they can't be clicked so you don't open the master layers
3. Set or toggle a variable on the master slide or layer that triggers the current slide to reopen the needed layer
4. Move the next button to the slide base but keep it hidden until the final layer is shown. That way it remains available