Hello all-- a few more notes, and progress.

The following didn't work:

Removing "USB Composite device" under windows device manager.

Passing through only the USB controller (not the other associated devices) and adding a passthrough.map entry , d3d0 mode, for that controller only.

The following does seem to work:

Adding the USB controller, associated security platform and "nonessential instrumentation" to the VM, and then using a wildcard with the vendor ID (which is the same for all devices)  in the passthrough.map. I was reluctant to try this for fear that the wildcard could nuke other AMD devices somehow (same vendor?) but the  VM now reboots happily with both USB and GPU passthrough, yay!

b