Not something you would do by hand.

Indeed, but I have to do it.

That I don't understand. Isn't the angle a result of a calculation when the position of a point is changed?

If you do so you will never have a plane or it will be very hard to align the vertices to be part of the plane without the help of the campskeleton calculations.

There's a simple and widely used solution: use triangle faces and don't try to force rectangular faces. Even rectangular faces are most likely converted to two triangle faces. Open an exported box in Blender and you will see that each side has two triangular faces and not a single square face.

Every part needs the outside borderline as the base

I've anticipated this statement. But now you give me a reason to disagree.
...
The real roof shingles are attached accordingly with the water flow natural direction. The roof builders know how the water flow and don't try to fight with the nature force. Sooner or later the Nature law's win. The water flow doesn't care how you arrange the texture in SH3D or how nice and symmetrical it looks in the program. Water simply flows in the direction it has known for millions of years. From top to bottom the shortest way.
The border orientation doesn't mater.

I agree. I probably stated it wrong but I meant the same. Not water flowing to the top [:))] but the direction of the texture. It should be towards the baseline (where water flows of the roof). With the new functionality of point editing that might change: texture direction towards the lowest point.

I'm currently experimenting with generating furniture parts, for example a cabinet door with handle and hinges. Throw in the dimensions, a bevel size, a material type (color/texture/glass/mirror), a material name, hingenumber, and if you want it smoothed and it generates obj/mtl files for a complete functional door that can open. It does one thing that's difficult to create in Sweet Home 3D: bevels. And one thing Sweet Home 3D can't do at all: smoothing. Shelves/sides were simple but the door was a struggle with keeping the handle and hinges the same size when the size of the door itself changes. Now I'm breaking my head over generating drawers. The goal is to generate parts for a cabinet in any given dimension, even if smaller than the 0.1 limit, like your angles.
So far it seems to work fine but I makes me completely understand your problems with calculating vertices/faces and textures, and of course many digits after the decimal point.