Arnaud,

In the 3D preview window you can choose between Display all levels and Display selected level. This may (partly) solve your problem.

Ideally, I think, you would make walls on any level not higher than the height of that level. In your case that means each level has 1.50 mtr high walls, interior and exterior. You can easily copy walls from one level to the other. Just select them, then with CTRL-C and CTRL-V paste them in the new level.

There is no way to slice a house vertically to make side-views.

Hans