Showing multiple transparent layers over base object

Two principles will help you here: It doesn't hurt to hide a layer that isn't showing. So I changed the triggers on the shirt slider. If it is one, it shows one shirt, if it is three, it shows the other, and if it is two, it hides them both. You can make those same changes to the other sliders.

The other principle is what you stated - The last chosen layer shows on top. So I gave the shirt layers some extra triggers to show the pants again, if the pants are already showing. That moves them to the top. You will have to do something similar with any of the layers you want to be on top. Decide what is supposed to cover what and under what conditions, and you can go from there.

For the checkboxes, I created a variable for each one, but I wrote the triggers only for the hardhat.