Update issue: "E: The repository 'http://www.debian-multimedia.org etch Release' does not have a Release file."

It appeared previously that I had trouble with some background update process (?) updating "snapd". This may have been a known issue, but it took ~10 minutes for my system to shutdown while it waited on this update to complete. Once this finished, I booted my system, and all icons were missing (e.g. for all applications) from my OS. I then tried to update my system. Whether this was attempted via the "GalliumOS Update" or sudo apt update, the result was the same:

Ign:12 http://debian-multimedia.org etch InRelease                                   
Err:16 http://debian-multimedia.org etch Release    
  404  Not Found [IP: 104.27.183.102 80]
Reading package lists... Done                     
E: The repository 'http://www.debian-multimedia.org etch Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

It appears that this issue is caused by the fact that the address of the third-party media archive 'http://www.debian-multimedia.org is incorrect. This archive was long ago moved to 'http://www.deb-multimedia.org.

There were notes on deb-multimedia.org posted this week (March 22, 2019) about a change to the address of the archive to "archive.deb-multimedia.org". However, I was unable to use that new address (404 error). Instead, I used "www.deb-multimedia.org". Also on deb.multimedia.org, I found posted previously that "etch" should be taken to mean "oldstable". With these things in mind, I made the following change to my sources.list:

Replace deb http://www.debian-multimedia.org etch main with deb http://www.deb-multimedia.org oldstable main

This fixed the 404 error above. That said, the following is unclear to me:

What the use of this media archive is
Why we are using a third-party mirror for this archive
Why a debian jessie archive is the correct archive for this (I understand that Gallium is downstream of Ubuntu is downstream of Debian, correct?)
Whether I should have used the oldstable release or oldstable-backport release (or none of these)

Next, the public key used to verify this package did not exist.

Err:7 http://www.deb-multimedia.org oldstable InRelease                                        
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5C808C2B65558117
Reading package lists... Done
W: GPG error: http://www.deb-multimedia.org oldstable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5C808C2B65558117
E: The repository 'http://www.deb-multimedia.org oldstable InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

I followed the advice here: http://forums.debian.net/viewtopic.php?t=133986#p675127 and executed the following: sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 5C808C2B65558117

This fixed the issue and I was able to upgrade, etc..

System Info

Hardware: Acer Chromebook R 11 CB5-132T Firmware: John Lewis

I believe I installed from an ISO, not running nightly builds

GalliumOS / galliumos-distro

Update issue: "E: The repository 'http://www.debian-multimedia.org etch Release' does not have a Release file." #497