Happy to see you back Pencilart [:)]

If you want to search in the library, you should select the Searchable list option in the preferences.

I'm not sure to understand what you mean by level transparency. At the moment, objects at lower levels are visible only if they are higher than the elevation of the upper level.
If you added the staircase in level 1 before adding level 2, you could have drawn walls at the upper level around that staircase, couldn't you?