1.) If using FileMaker to access the database, OnLayoutEnter is a script trigger that will server. But for IWP with a browser, you'd need to set up navigation buttons that performa a script and then the code to hide/reveal can be included in the navigations script.
But given the differences between Browser clients and Filemaker clients, i think a separate set of layouts for each user group makes more sense.
2) I know of no way to make a specific tab panel disappear. You could make the entire tab control appear/disappear by putting it inside the panel of another tab control but not just one tab out of several.
I think creating different layouts for different platforms is probably going to work best as well.