OK, I answered my own questions. Upon looking at the drawing in AutoCAD, I discovered that I had actually linked a DWG that contained a real Xref. Since ArchiCAD didn't complain, I assumed the DWG was self-contained.
The part that I was trying to Explode was indeed an embedded Xref, and though it did seem to Explode, it created uneditable layers. THis is still a bug, but it has a work-around. The solution was to delete everything by deleting the layers in the Attribute Manager, and then bring in the ACTUAL DWG file that I wanted (which had been Xreffed) as a Drawings, and Explode that. It worked as expected.
So the moral is, beware of AutoCAD files with embedded Xrefs!
Chuck Kottka
Orcutt Winslow
Phoenix, Arizona, USA
ArchiCAD 25 (since 4.5)
Macbook Pro 15" Touchbar OSX 10.15 Core i7 2.9GHz/16GB RAM/Radeon Pro560 4GB