Going back to offsetting a single edge in SketchUp in case someone else comes along with a similar question. Offset requires a plane of reference so the tool knows which way to do the offsetting. With a curve such as an arc or even just a couple of edge segments that’s not a problem because the points on the curve define the plane. With a single edge segment as you were trying to offset, there’s nothing to define the plane of the offset in 3D space. You seem to the trying to apply your 2D CAD experiencce to the 3D modeling. Don’t do that.
Thanks so much. My cad experience was 3D and I’m wishing there was a way you could toggle through the different planes. I still haven’t figured out how to rotate something in the YZ plane… but I’ll keep looking.