Installing the Xen PV Bus Driver from (xenbus.tar) alone leads to an unbootable system. So that seems to be the problematic driver.
Installing the Xen interface driver (from xeniface.tar) allows a successful reboot and installation of QWT version 4.1.69. QWT installation is successful, too, if only the necessary Xen PV drivers (interface and PV Bus) are selected, and results in a working system with working copy to/from other qubes, but without seamless mode, as is to be expected. In this system, however, the Xen interface driver is installed twice, both the 8.2.2 and the 9.0.0 versions, and I suspect that QWT simply uses the 8.2.2 version.
If the Qubes GUI agent is installed, either at QWT installation or later on, the system hangs at shutting down and has to be killed. and cannot be recovered. If, at the next boot, it is started in protected mode, it hangs at loading the CLASSPNP.SYS driver, then continues and crashes with the message qrexec-daemon startup failed, well before any qrexec timeout.