It's an good option to lock levels with all the items not only the base plan.
I use the same strategy (separate level) for emergency 2D symbols.
I've try Your modified version but I need more exercise or more indications on how it work.
Suggestion: add an contextual entry under level tab to lock/unlock current level or all levels except current.
About the view of 2D symbols in plan:
-I've set the elevation to 10cm BUT could be elevated how high You want.
-Wile are symbols and appear only in 2D plan they can not be used like 3D objects. On my apartment I have plugs and switch at the same x&y but at different z.
Is useless to overlap this symbols because I can distinguish nothing.
-You can select all, Ctrl+x, Ctrl+v in the same place and will appear over other furniture. The last inserted is the first view.