As you observed, only wall guides of the previous level can be seen in the edited level, or at the lowest level guides of its upper level. The idea is that you draw a level, then need guides in an upper level, or in a basement.

having some attributes in the level settings like "visible" [...] would be simpler

It will make the program too complicated, because you would have to detail also in which other levels you want a level to be visible, otherwise you would get many guides for houses with multiple levels.
You could simply copy paste walls from a level to another one and remove the ones you don't want to keep, or program you own version, as you decided to do.

Therefore the order of adding levels impacts behavior.

No, you missed the fact that levels are always ordered from the lowest level to the highest, then at the same elevation, in the elevationIndex order set in the level modification dialog with the arrows at the right of Levels summary list.

the current code violates UI first principles

What principles do you have in mind?