Debian -- Details of package linux-image-cloud-amd64 in stretch-backports - can consider. Could maybe download from stretch-backports and add to Whonix stretch repository. Could you manually test please if it works?
Also need Debian -- Details of package linux-headers-cloud-amd64 in stretch-backports.
Does Debian -- Details of package linux-image-cloud-amd64 in stretch-backports break Debian -- Details of package virtualbox-guest-x11 in stretch-backports? Could you manually test please if it works?
(I didn’t mean to suggest for Non-Qubes-Whonix to use the very same kernel package (rpm) as Qubes. While I don’t want to preemptively exclude it, I didn’t consider to research that. I guess alien will never be good enough and could go horribly wrong for a package as important as the linux package. Horribly wrong: missing maintainer scripts, kernel post install scripts, other incompatibilities Fedora vs Debian, breaking boot)