This took much effort, many retries, exploring many dead ends, over three days to find. The delta between Debian and Devuan and Virtualbox can get messy if any one of the three is not aligned. In the end it turns out to be quite simple, although there are a couple assumptions involved to keep it that simple.
Prep the installation with the correct kernel headers
After you install Devuan as a guest from an ISO, get to a bash prompt (Terminal) and get to root within the VirtualBox guest. Then do the following, as root:
apt -y update apt -y upgrade #get the latest linux image apt -y install linux-image-amd64 #get the headers that go with that image. this takes awhile, be patient apt -y install linux-headers-amd64 reboot
Ignore the 'rcvboxadd' notice that it will say you can do instead of reboot, or you'll get an error later. You just upgraded the kernel; it doesn't happen often, so kindly reboot. Note the command to install linux-image-amd64 above. This is the one line that many online how-tos miss. If you don't do this and the existing image is even slightly different from what's in the security repo, nothing else works right. The kernel and the headers need to match.
Install the Guest Additions
After reboot, go to the Devices menu of the guest. Right-click "Insert Guest Additions CD image," then do the following in the guest bash shell, while logged in as root:
mount /dev/cdrom /media/cdrom cd /media/cdrom ./VBoxLinuxAdditions.run
When that finishes, logout and then login.
That's it!
Assumptions
Now for a couple assumptions: If you have multiple VirtualBoxes you might not always have the correct version of Guest Additions ISO embedded in the host application (i.e. if you upgrade the host, you often have to upgrade all guests to match, and that's not always what you want to do). Download the ISO that matches your desired Host version (try "vboxmanage --version") here:
https://packages.debian.org/sid/all/virtualbox-guest-additions-iso/download
Then mount that version instead of the ISO image which appears under the Devices menu.
Another assumption: You should have an /etc/apt/sources.list that looks something like this, the key one here is excalibur-security, but the rest are useful unless you specifically don't want them:
cat /etc/apt/sources.list deb http://deb.devuan.org/merged excalibur main contrib non-free non-free-firmware deb http://deb.devuan.org/merged excalibur-security main contrib non-free non-free-firmware # excalibur-updates, to get updates before a point release is made; # see https://www.debian.org/doc/manuals/debian-reference/ch02.en.html#_updates_and_backports deb http://deb.devuan.org/merged excalibur-updates main contrib non-free non-free-firmware # excalibur-backports, previously on backports.debian.org deb http://deb.devuan.org/merged excalibur-backports main contrib non-free non-free-firmware
The astute will note that I did not need to use Sid (unstable) to get this working, although if this doesn't work for you right away, you may want to consider that approach to get the correct virtualbox or other versions aligned.
Helpful links
https://wiki.debian.org/VirtualBox?pow_referer=#Installing_the_Guest_Additions_on_a_Debian_VM
https://packages.debian.org/sid/all/virtualbox-guest-additions-iso/download
https://itslinuxfoss.com/install-virtualbox-guest-additions-debian-12/
https://www.reddit.com/r/virtualbox/comments/12vkjwk/where_to_download_guest_additions_iso/
https://askubuntu.com/questions/299415/update-linux-headers-when-kernel-is-updated
https://linuxconfig.org/install-virtualbox-guest-additions-on-linux-guest
https://terokarvinen.com/2021/install-debian-on-virtualbox/
