I've read several articles on getting the sketch transform and transforming sketch coordinates/points to model coordinates - none are working as expected.
The transform returned by ISketch::ModelToSketchTransform does return the correct plane, however the origin is not where I would expect. It also doesn't transform sketch points to model space correctly.
Sketch Origin issue.png illustrates the problem.
The yellow/orange X/Y arrows show where I would expect the origin to be. Something like (0.11, 0.05, 0.05). Instead I get (0.11, 0, 0) which is where the global X axis intersects the sketch plane. I'm assuming this the model origin projected along the plane normal, also the standard geometric form of a plane.
I'm missing some key piece of information or assumption that others take for granted.