Well. I'll tell you the full story (and I beg your pardon for its length).
My final target was to check & test the QEMU/VirtualBox-VMUB (Virtual Machine USB Boot, the DavidB's tool).
It's worth saying here that the direct trials with the VirtualBox-VMUB combo went fine, just at first shot.
Instead, for the QEMU-VMUB case, before going directly with the combo trials, as an intermediate step I preferred to check (separately and independently) the QEMU Manager functionalities, since I'd never used that tool.
Unfortunately (but only later I realized this was my innocent fault), the bootable UFD I used for the first trials was based on a WinPE for Windows 10 (64 bit) and so in no way I was able to boot that UFD.
It was at this point that, having the feeling that the installed Qemu Manager was using a very old qemu.exe version (therefore, not compatible with the "advanced" WinPE), I decided to look for some updated executables and so I installed the last version of Qemu for Windows tool (from Stefan Weil Page).
But, after several tests (with negative results) it was clear that I should have asked for some help to the Reboot experts, opening this thread.
Then, once I read your post, before proceding with your advices, I realized the idea about the "too much advanced WinPE" and therefore I built another bootable UFD (but based on a more "standard" WinPE 3.1). This time, QEMU Manager was able to boot the new UFD, even if very, very slowly...
Finally, hoping to improve/extend the capabilities of QEMU (in order to boot more advanced WinPEs), I followed your advices making the last tests and trying to extend/translate the commands from Qemu Manager to Qemu for Windows, but this is a well known story...(described in Post #3).
Therefore, to make the long story short, I'd say that the main problem I had with the old qemu.exe (coming with Qemu Manager) seems to stay on its "limited" compatibility with "newer" WinPEs/OS.
Regarding the version of qemu, I was not able to determine it for the Qemu manager I have tested. What I can say is that the Qemu Manager version is the last official one (7.0) and the properties of qemu.exe are :
Dimensions : 2.010.112 bytes
Therefore, if you think yours is newer, please send it to me, possibly together with some companion notes/documentation.