Thanks again for your help, Michael. Yeah, I went back and did try it with joined surfaces, so I suspect I was doing something wrong in the first video. In fact, I suspect I was SELECTING the back face and had deleted the 'trimmed' face, so the offset was taking place on the back. I'm sorta used to not being able to 'see' surfaces (polys) with normals pointing away, so that probably explains my confusion. Might be something to add in the future (hide back faces).

And FWIW, I think it might just be best in the future to create a cutting object for the groove, that way I think I can get even more detail in my path (not have to worry about radius offsets not working out)