You should be able to boot the dvd, go into rescue mode, get a network connection and install grub-efi-amd64. You don't need the -signed version unless you're using secure boot.

All the installation media (including desktop-live) boot with grub on uefi and boot with isolinux on bios. If you can't tell the difference by looking at them, you can test:
Press TAB to edit the boot command in isolinux.
Press e to edit the boot command in grub.
Press ESC to return to the boot menu - this one is the same for both.

If you're already booted into the system, check to see if /sys/firmware/efi exists. If it does, you booted in uefi mode.

Gparted leaves 1MB free at the beginning by default. If that's not there, it could be the reason for grub failing. Open gparted in the live session to check that it's there.

The log is in the user's home directory on the hard disk.

mount /dev/sda3 /mnt
less /mnt/devuan/refractainstaller.log

Is there something unusual about your setup? How did you partition the drive? Gparted? Something else? Is there 1MB free space before the first partition? Does it use gpt partition table? What's in the log around the grub-install command?

It sounds like you made the right choices for a legacy bios system with msdos partition table. The right place to put the bootloader is /dev/sda, so it should have worked.

What error message did you get? You can check the installation log by booting the live media, mounting the home partition and getting refractainstaller.log from the user's home. The grub section should be about three-quarters of the way down the log. Look for an error message about grub.

The other way to do the install would be to install the grub-pc package before running refractainstaller, but do not let it install the bootloader at that time.  Run dpkg -i /grub-pc*.deb

After installing the package, you can install the system to hard disk, and you will be given a choice of where to put the bootloader. Choose MBR of /dev/sda. When the installer gets to the grub section, you will see a button that says 'Install bootloader' in the same place where you saw 'Copy files' the first time.

I don't see any problems with the hard disk.

Run the head command again, but try '-n 2000'

See if you can get a few lines beyond the point where xorriso runs. (about 20 lines past it should be enough) Look for something like this to find it:

xorriso : UPDATE :  0.23% done
xorriso : UPDATE :  3.28% done
xorriso : UPDATE :  11.45% done
xorriso : UPDATE :  19.93% done, estimate finish Wed Dec 25 22:11:41 2019
xorriso : UPDATE :  28.85% done, estimate finish Wed Dec 25 22:11:40 2019
xorriso : UPDATE :  34.11% done, estimate finish Wed Dec 25 22:11:41 2019
xorriso : UPDATE :  36.97% done, estimate finish Wed Dec 25 22:11:42 2019
xorriso : UPDATE :  40.51% done, estimate finish Wed Dec 25 22:11:44 2019

I have no idea how you managed to get an 18GB log file. Try this...

head -n 1000 /var/log/refractasnapshot.log > beginning.txt
tail -n 1000 /var/log/refractasnapshot.log > end.txt

That will put the first 1000 lines and last 1000 lines into text files.
Then try opening them with a text editor and see what's there.

That site isn't working for me. If I turn off noscript and allow all cookies, I see the name of a video file. I don't see a text file.

I'd like to see /var/log/refractasnapshot.log. Send it to me in email or paste it somewhere and give me the link. Or read it and see if there's something obvious. I wouldn't expect the decompression error on the initrd to cause a 0-byte iso to be created.

Edit: I just realized you ran the non-gui version, which doesn't make much of a log file unless you run it with the debug option. (That happens automatically with the gui version.)   Run sudo refractasnapshot -d

Make sure you didn't run out of hard drive space. You might need to exclude some things to make the system copy and the squashfs smaller. (3.9G is pretty big.)

In the past, netinstall isos would give you a small and functional base system if you installed without a network. The current netinstall isos won't do that. For an offline install, people should use the server or desktop installer isos or the live isos.

dev-1-dash-1: Thanks for the feedback. We have our wizards working on it.

dev-1-dash-1:  I don't know if you can use jessie to debootstrap beowulf. I would avoid using 'testing' because debian and devuan testing are not the same right now. It might cause problems.

I also don't know if you can install beowulf's debootstrap in jessie. Can't hurt to try.

I do know that all the devuan live isos have debootstrap installed. You could do it from a live session.

kapqp: I see only two lsb packages - lsb-release and lsb-base. They both get installed with the default desktop. The minimal-live just has lsb-base.

Make sure build-essential is installed. (Yes, the package name is singular.)

I think I saw the missing hostname issue once. Didn't happen with most of the installs I did.

Size of the iso was a concern, and the graphical installer takes up a lot of space.