How I got apt update and install to work (with Debian Jessie retired)

OK, for those still struggling to follow the whole thread or having difficulty with VI or trying to use a Windows editor, here is a summary and quick solution:

This is my current /etc/apt/sources.list with firmware 6.10.9:

deb https://apt.readynas.com/packages/readynasos 6.10.9 updates apps main

# deb http://mirrors.kernel.org/debian jessie main
# deb http://security.debian.org/ jessie/updates main

deb [trusted=yes] http://archive.kernel.org/debian-archive/debian jessie main contrib non-free
deb [trusted=yes] http://archive.kernel.org/debian-archive/debian jessie-backports main contrib non-free
deb [trusted=yes] http://archive.debian.org/debian-security jessie/updates main non-free contrib

/etc/apt/apt.conf remains unchanged for my original post:

Get::AllowUnauthenticated true;
Acquire::Check-Valid-Until false;

Now, if you are having trouble getting them this done, here is a solution. Note that these files are from my personal DropBox, so may disappear after a few months.  You still need SSH, but don't have to edit anything.

https://www.dropbox.com/scl/fi/zoqxg8u1y0ztqjkfrvod7/sources.list?rlkey=5ouaymaz3ejn0ssc58ad8nfrk&dl=0 

https://www.dropbox.com/scl/fi/25hs7nn12830vz9xk2phx/apt.conf?rlkey=maf95f9ncug9q09v4znm0kiwt&dl=0 

Put these somewhere on your NAS using SMB, the GUI, or whatever. Then, from SSH, you simply need to copy them from where you put them to their proper place. Then do an apt-get update (Note: NOT upgrade!). When you do so, these are the only warnings you should see:

W: GPG error: http://archive.kernel.org/debian-archive/debian jessie-backports InRelease: The following signatures were invalid: KEYEXPIRED 1587841717 KEYEXPIRED 1668891673
W: GPG error: http://archive.kernel.org/debian-archive/debian jessie Release: The following signatures were invalid: KEYEXPIRED 1587841717

Well, those are the only ones unless an app has put something in /etc/apt/sources.list.d, in which case you may have to add [trusted=yes] there as well.

And once you get this done, run apt-get install nano to get yourself a usable editor.  If you are sticking with ReadyNAS, there's a good chance you'll need it down the road.