And yet further progress--

The 1070 Ti with the error 43 was also dead in a single board system (board problem).

Was able to passthrough Asus USB Bluetooth dongles as USB devices (not as PCI devices; done through a non-passthrough USB controller-- the very same 3.1 controller that can't pass through anyway) to VM's, which could then attach Bluetooth keyboards and mice.  However, I'm going to stick with Virtualhere because I'd rather not deal with the input lag and batteries, and can live with making some vm's dependent on others.

Passthrough of the onboard audio worked fine despite not passing through other items in the IOMMU grouping (like a SATA controller). I've never actually tested or used the sata controllers, NVME works fine.

The on-board Bluetooth was eventually discovered appearing as a USB device and could be passed through ("Intel USB device") but not actually used in the VM (recognized as Bluetooth controller but had an error, "wouldn't start.")