I finally figured this issue out, and want to report it so otherscould benefit and also to be fair to ZWO - the issue was not primarily a light leak with a ZWO product. Unfilled screw holes on the front of the ZWO filter wheel were causing some issues, which were fixed with some electrical tape... but this did not fully fix the issues. After extensive testing, I found no other light leaks involving a ZWO product. This included the junction between the camera and the filter wheel and around the filter wheel motor.
Extensive experimentation with a lot of configurations and spacers finally revealed that internal reflections from my Stellarview field flattener was the issue. Again, to be fair, this flattener was sold as being optimized for DSLR cameras (for which it worked great for me). I started using is with the ASI2600 and ZWO filter wheel, proper back focus, but perhaps because of the different internal geometry it was causing weird effects. No hit on Stellarvue, they state that it was designed for DSLR, so no one to blame here.
Just goes to show - every rig is different and getting every piece of equipment to work with every other piece still takes some thought and work!