There is no need for any init script for simple applications like firefox(-esr), libreoffice, vlc, virtualbox ....

LightDM is a good choice. I am using it on Mate or Cinnamon since many years now.

Well, currently neither Bookworm nor Daedalus (5.0.0 and 5.0.1) will run from a Ventoj stick ok. There have been changes in the graphics that are incompatible with Ventoj.

I have seen a claim that using the grub-mode might be a workaround, haven't tried it yet.

Older versions are fine.

Edith: My mistake, confused Bookworm behaviour on Ventoj with boot issue in Virtual Box and efi mode. Bookworm works fine from Ventoj.

Some comments to a dual boot with Windows.

First: Windows should be installed first and sets the boundary conditions for the partitioning scheme of the system disk. Legacy install means MBR partitioning, UEFI install means GPT partitioning. Valid for Win7 and newer. My advise for going on to use XP: copy it into a VM.

Second: Linux systems don't care about MBR or GPT, legacy and efi, installation is possible in any combination. But side conditions do apply:
- for a legacy install on a GPT disk you will need a bios-grub partition with no file system on it.
- an uefi installation requires an esp. Size somewhere from 128M to 512M if you are generous, FAT32 as file system is mandatory, best is if its the first partition on the disk, esp flag set.

Some side effect of the os-prober: it can't find other encrypted Linux installations. Manual work is required to add such a system into grub.

You know how to call up a console? And how to gain superuser rights? Here is a rough guideline:

$ su -
# cd /etc/default
# nano grub
# update-grub

Which Login Manager are you using? LightDM, sddm, something else? The Login Manager should give the choice.

This database isn't present on my main computer (only LAN), but on my laptop its installed as well in Chimaera as Daedalus.

If I am not mistaken the wireless-regdb is installed with wifi drivers and firmware as a dependency. At least this was happening when installing the laptop. I guess there are dependencies.

Look into Ventoj.

Is fuse installed?

May be simply due to an issue with the sources.list. New in Debian 12 Bookworm and in consequence in Devuan 5 Daedalus:

There is a new sub-repo called non-free-firmware. Look i.e. into this thread: https://dev1galaxy.org/viewtopic.php?id=5688

To the technical side of things:
1.) Devuan/Debian can be installed in legacy or efi mode on MBR or GPT drives in any combination when Windows isn't involved. Well, some side conditions do apply here and there like for efi installs you will need an efi partition in any case ... but its your choice which way to go. On new drives I suggest to use GPT. Not to count partitions and care about logical drives really is some progress and much more logical.
2.) In your case do not use any automatism. Make sure you use manual partitioning. That way you can select exactly where to install Devuan. Though a backup never hurts when things go south by chance.
3.) I think when you add the existing swap partition the installer will format that again. AFAIK you don't have the choice not to do that. If that swap is used somewhere else you will have to modify the uuid in the fstab.

What a rant without telling anything about the PC hardware involved ... Ok, some comments anyway ...

Why Beowulf today? Is that a very old PC that does not work any more with latest versions? If you try to install in efi mode, it must be something relatively recent, so Daedalus should be fine.

efi-Mode and not writing to the efi partititon: any Debian/Devuan installer needs to be booted in efi mode if you want an efi installation. Make sure when the boot stick is selected that you use the one that is marked "efi"! If not the installer will try to write a legacy grub. This may explain this error 127.

Installations from live media often suffer from various problems. My advise: Use a regular installer. As an experienced user I do prefer the netinstalls, and I only use expert mode and manual partitioning.

To make sure only one efi partition is used, either remove all other devices from the PC, or use manual partitioning to select the efi partition to be used if more than one exists on the PC. Do not format it!

BTW: there is no fixed device detection order. Normal boot and installers can change detection order by chance depending on kernels used, timing, chance ... Always have a very close look before letting go.

Also the swap partition can be set during manual partitioning, but AFAIK the installer always will try to reformat the device. You will need to set the correct UUID in the affected fstabs manually.

Theoretically you can attach an existing /home during installation also by manual partitioning. But this is prone to errors, you have to make sure that you do not reformat the device. My personal advice is to attach it by modifying the fstab after you booted the install successfully, this would be my way.