maker:

Of course, if you want to add openings or modifications in the wall, it could be a bit of a problem.

Yes, that's true. But, seen that this trick is so simple, you could easily make the changes in the original and then make a new copy.
If you need to make a simple hole in a wall, you could insert an invisible box in the wall. To change its colour or texture, you can use a thin box.

As too layers, although there hasn't been an official announcement, Emmanuel has hinted several times that a future version of SH3D may include layers.

And, yes, I agree that the development of the program is lightning fast!

Hans