Not something you would do by hand.

Indeed, but I have to do it.

But you don't need to if you can adjust the vertices coordinates.

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.

It will also open up possibilities for dormers.

I already wrote (a year ago) a tutorial about this.

I can't see it very well on the image but the texture alignment seems to need some fixing.

I've posted the file. You can't modify the roof with your current plugin but you could set the texture.

Every part needs the outside borderline as the base

I've anticipated this statement. But now you give me a reason to disagree.

Just like real roof shingles would be attached.

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.

But, most probably I'm (as usually) wrong.