Thank you, DannyT. That's quite generous of you.

As of yesterday, I'm on a crash course, trying to learn the basics of ViaCAD Pro for Mac. It was recommended that I might try that, in order to see whether it might be able to process the STEP file better. And it does. No geometry issues, and it even seems to have slightly better structure, too. I have no idea why it would be that much different from MOI, but it seems to be so far.

I also installed the V3 MOI Beta. Michael G - that DID seem to fix most of the bad geometry. There is still one odd spot, where the surface has a hole. But it's much better than what happens when I bring it in with V2.52.

I have a suspicion, that I might just need to use both MOI and ViaCAD in my model prep pipeline. I'm becoming extremely comfortable with working in MOI (and I LOVE this fantastic help forum - simply nothing else like this!) so I might try processing the STEP files in ViaCAD, and then doing the rest of my work in MOI. Plus, ViaCAD seems to be lacking any control over the final .OBJ output (which is crucial) and both MOI and Rhino have fantastic mesh generators.