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.

You need to inverse the transform returned from ISketch::ModelToSketchTransform using Inverse method as you need sketch to model, not model to sketch.

Why do you expect the origin to be at that point. The origin (0,0) of sketch is as one below: