I suggest a solution that would address both what @sjb007 said and your proposal regarding the Y-shaped staircase.

Of course, there are still some unresolved issues, such as the fact that it’s still impossible to create external connections for complex-shaped ceiling cornices. This issues was (partially) solved. Read this posts:
Baseboards (first attempt).
Moulding plugin (second attempt) and Moulding ...

Also, at Plug-in developer’s guide fix Figure 13 and Figure 14 on Model classes architecture section.

Sorry for double post.
Tools menu IT IS NOT a default menu item.
It appear only if you install at least a plugin which programed it(Cabinet maker, Roof generator, Advanced editing etc).
Install Cabinet Maker and you will have Tools menu after you restart the SH3D.

Tools menu IT IS NOT a default menu item.
It appear only if you install at least a plugin which programed it(Cabinet maker, Roof generator, Advanced editing etc).

Moi, j'aime bien ! À notre âge, à ma femme et moi, on n'a pas besoin de plus.
Tu as raison pour l'entretien.

My final dialog with one of the AI after few days of trying and fail.

Q: With TRIANGLE_ARRAY shapes work as expected but with QUAD's don't.
Could QUAD_ARRAY to not be proper solution for modeling shapes?

A: Yes, this is a fundamental representation problem. QUAD_ARRAY is the wrong primitive for ...

Don't rush—give this version a try.
I would like you to fix the issue with face grouping when multiple adjacent faces are selected (as was the case in my version with triangles).
This is especially necessary for the extrude operation. The newly created faces should enclose the entire selection, not ...

@Xlance
I'm not going to argue with you. The Outset and Inset buttons don’t do what they’re supposed to ...
What are they supposed to do? (Scaling is done by dedicated x, y, z inputs) This option wasn’t in your previous version.
Even so, Outset/Inset were supposed to perform uniform scaling ...

There's no rush.
I was impressed by the mathematical models (69 of them).
I also liked how the loaded objects look.
You did an excellent job! Still, if you can't beat them, join them [:)] So I'll collaborate with you if you agree. [:">]
Here is a temporary version you can use as a starting point ...