Thanks your questions.
1) Can a nonplanar curve be offset?
I think "offset" you mentioned is sketch offset. Sketch offset only can offset sketch curve. Here, the sketch is 2D sketch, not 3D sketch. Fusion cannot support 3D sketch now.
2) Can a compound planar shape, e.g. a rectangle with a circular hole, be offset?
Yes, it can offset compound curve. You can have a try.
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