I think the best way is using live debian image with Wayland cage. User can change something, but it will be lost on restart. Debian supports plymouth out-of-the-box if enabled in grub.
In this guide author dumped edid from disaplay and then loaded it without any modifications. So after reloading edid you can choose any resolution? Or I understood something wrong?
User sddm also cannot execute poweroff (it is ok) but if I press shutdown button in sddm it will poweroff (not ok).
If I press shut down in KDE Plasma I will get black screen and no sddm (I can restart it with systemctl restart sddm from tty and it will work again)
How can I fix this bugs?
No, this is not possible. Virt-manager is just a GUI over libvirt. For now libvirt doesn't support USB passthrought by port id, only by device id (which changes on each plug) or vendor id.
You can use regedit on Linux via wine!