Me? Idiot? Help! And I that soo admire unarticlated, aggressive one-liners!

In your post #7, it looks like your attempts with "su" fails because "ash" is not found, which suggests that "ash" is wanted to be used by the initial file(s) that are loaded at login, but is found missing. It looks odd, and maybe you should check up on that first, i.e. specifically why do you get the output:

bash: ash: Kommando nicht gefunden

Just in parenthesis, regardless of your recent experiences, the program su works perfectly fine in beowulf, and it is directly brought in from debian buster. Though, it works differently from how it worked some time period ago, when its juggling of environment variables was different.

Thanks @berni51. I now got the German translations installed.

Hmm wmctrl -l gives me a list of all windows across all workspaces, detailing window id, workspace number (with a - in some cases for some reason), host name(?), and window title.

@danista: thanks. Afaiui, that boot entry says (to grub booting)

That's a different thing from boot-up using the ISO kernel and initrd on partition 2.

It might possibly work if you copy the kernel and initrd from the ISO, where they actually reside in its /boot/isolinux/ directory, to replace the ones you have under /boot on partition 1. You should do that after having otherwise set up partition 1 with a full grub boot including that menuentry to identify partition 2 as (post-pivot) root file system.

I suppose this still brings a period of confusion for the kernel upon pivoting to the root file system, as it results in shadowing the partition 1 /boot tree, but that shouldn't cause an issue unless you then try to install onto partition 1 (or partition 2, of course).

Hey, friends; no need to get defensive and personal when the OP has taken the trouble of putting their mind to articulating how their experience differed from their expectation. Be positive: the OP wants to help.

Yes. I'm just troubled by, again, a utility not having a proper man page, and secondly that proper documentation is not installed together with the utility. All sorts of other places of documentation is good (the man page would point them out). But all that is of course just one of the "wheels" that needs to be regularly reinvented and done differently, either just because it's possible or because the controlling generation doesn't bother to learn. It's a personal beweiling, basically irrelevant for the topic, so I'm not sure why I keep going with it.

Hopefully the @OP now finds it resolved.

It seems grub is one of those packages where it's stupidly hard to find documentation, with man pages that just say that "this is documented somewhere else and I'm not telling where"... it turns out that you need to have grub-legacy-doc installed, which provides the texinfo file with documentation using the info system.

Anyhow, according to that you should also use

GRUB_TIMEOUT=0

Afaict GRUB_HIDDEN_TIMEOUT is the time to wait before showing the menu, and GRUB_TIMEOUT is the time to wait (in addition) before booting to the default choice.

If you also have

GRUB_TIMEOUT_STYLE=hidden

then the menu is not actually shown, but the time logic remains the same.

According to my /etc/inittab my boot control script is called /etc/init.d/rc, and that is a link to /lib/init/rc, which is a shell script that by the looks of it has a great number of ways to avoid concurrent boot.
For example put concurrency=none on the boot command line.

I use google-chrome-stable for jitsi myself so I'm not to familiar with that interface, but that toggle it shows seems to be regarding the output and not the input?

I also don't use apparmor, so can't help you there. Not much use I'm afraid; I'm sure you checked through all the browser's own "permission settings".

Yes, the Devuan key was changed a fair while ago, and the easy way out would be

# apt-get -y install devuan-keyring

That package drops a couple of files:

/etc/apt/trusted.gpg.d
/etc/apt/trusted.gpg.d/devuan-keyring-2016-archive.gpg
/etc/apt/trusted.gpg.d/devuan-keyring-2016-cdimage.gpg
/etc/apt/trusted.gpg.d/devuan-keyring-2017-archive.gpg
/usr/share/doc/devuan-keyring/README.md.gz
/usr/share/doc/devuan-keyring/changelog.gz
/usr/share/doc/devuan-keyring/copyright
/usr/share/keyrings/devuan-archive-keyring.gpg
/usr/share/keyrings/devuan-keyring.gpg

with the first three being for apt.

You should rather use the "server" iso, which is intended for offline installation.

In fact, all the beowulf installation iso have the same basic installer, but they differ in their on-media pool. The netinstall pool is just enough to let you install from the network, and if you don't, you don't get a very useful system. For beowulf beta3, now available, the netinstall iso pool has been inflated beyond bare necessity to correspond to a debootstrap minbase package collection. That is still not really intended to be used for an offline installation, but it might be possible.

Offline installation is rather what the server iso is intended for, or the desktop iso, if you want a desktop environment but don't want to juggle the CD sized expansion isos for the server iso. Both server and desktop also allow network mirror during installation the same way as the netinstall iso, in which case of course the whole package pool is available.