Hey Michael,
thanks for this big answer!
"So therefore the surface normal that a surface offset will go along will be like this"
Ok, I understand that, but I need the small surfaces straight. This object should be a rain hood for bull's eye. so I need a straight side to be able to glue it down, I´ve got a picture for you. I want to build it with a 3D printer.
Is tehre any possiblity for that?

"And it may be better to directly model the inner surface with its own set of curves rather than generating it with a surface offset."

I´m not sure what you mean.. Actually I built that solid ffrom a surface with ´shell´. Firstly I tried to built it with profiles and networks. When I joined them in the end, I just got a joined surface every time..

On the other pictures, I tried to trick out the normals, but that did´nt work :D

I´m looking forward for your answer:)

-Koi