- Set the transparency to 50%;
- Uncheck the wireframe checkbox;

Yep, with the wireframe box unchecked it works.

- It definitely needs the same reload as the roof generator

This means that the objects must be special furniture, just like the roofs. Maybe sometime in the future.

I don't understand in what way it has to be a 'special' object. It would be perfect if a 'normal' obj definition could be read, i.e read all the vertices and faces (like importing furniture.)

- The current preview is perspective. This means that you can't drag a selection rectangle to select faces on the front, it will select everything. In some future version it would be very nice to have an orthogonal view (flat) and only select the visible faces with the selection rectangle and not the ones behind it.

You're right, but it's pretty much the same with AOI. I don't know how it works in Blender.

In Blender you can switch between perspective and orthogonal view. You can multi-select (or unselect) everything that is visible in the view. That means you don't select vertices/faces that are hidden behind the front faces. Only a part of a face needs to be visible to be included in the selection. Besides that you can select/unselect everything by material name. What is very nice is that you draw a free-hand area for (un)selecting, not a rectangle. The selection is not lost when you click outside the area so you can draw a selection area multiple times to get everything you want to select.
An example with a sphere in the work area: Draw a selection around the sphere and select faces. Delete the selection, what is left is half a sphere.

Allow to add/remove materials. (Even if a new material is not used yet.)

Okay, but that might only apply to the current session of SH3D. The feature that lets you name faces was inspired by you.

And an important feature. For now I need Blender for that or manually edit the obj/mtl files. If you get so far as being able to read an existing object than a material pulldown can be populated with the materials found in the object definition.
In the current situation you can add to the pulldown when a new material name is assigned. After that you can select a material name to select all faces with that material name. Keep the selection and you can select another material name and add it to the selection (or assign it to a new name or another name in the pulldown.) Of course the pulldown can also be a list (Blender has a list, not a pulldown)

- Add smoothing/flat for the selected faces. Don't invent the wheel again, the code for it is out there in multiple versions.

Where? Give me some directions.

Search for "mesh smoothing code" and you will find multiple sites that either explain the process, have actual code, or point to libraries on github/sourceforge that allow you to implement smoothing.
Start here: https://nosferalatu.com/LaplacianMeshSmoothing.html It gives a good explanation and examples (C code). You'll be surprised how little code it takes.

A suggestion: make sure you completely license this to YOU with mandatory acknowledgment.

How? What else do I need to do?

I stated that wrong, GPL3 is fine but you should mark all of YOUR code as copyrighted by you. In several plugins I have seen user created code where the copyright is stated as spacemushrooms where it should be the creator. Only the libraries used from Sweet Home 3D should be left untouched. The template to start a new plugin is a template to create YOUR code and should have your copyright. Having the copyright is important because it makes you the owner and gives you the right to change to a different license later on if you wish so. You might want to investigate a dual-license to prevent commercial use in closed source like the new Sweet Home 3D has become. I think all of the advanced plugins should do this.

It's all work in progress. Keep it up, you are doing great!