This one is partially solved. Skip to the bottom if you're in a hurry.

mini.iso (2018-12-31 amd64) works on multi-boot usb without any special
commands - it doesn't need to find the cdrom. That was a nice surprise.

Expert install on uefi worked without errors. Installed only standard
system utilities.

System boots to grub prompt.

Booted from grub command line, ran 'grub-install' and 'update-grub' and
it still boots to grub prompt. os-prober is finding all the other
systems, grub.cfg appears to be correct and the bootloader is created on
the efi partition as expected.

Booting from grub command line and using the 'configfile' option works.

There's a grub.cfg in /boot/efi/EFI/devuan that uses the configfile
option, but it doesn't seem to be working there.

No errors around grub in the installer syslog.


Oh! Line from that grub.cfg:
configfile $prefix/boot/grub/grub.cfg

On the grub command line:
grub> echo $prefix
(hd0,gpt1)/EFI/debian

Solution:
If I manually boot into the system and run:

grub-install --bootloader-id=debian

Then it works. My devuan bootloader dir is called debian. :(

Anyone else run into this problem? How do we fix it?

fsmithred