I don't think that there is/was a "Devuan" version, because it is not supported in debian. 
How did you install it?  Was it with the repo mentioned here: https://wiki.debian.org/Opera ?

I would suggest to uninstall the "broken" version that you have and download and install the older version.  I am currently booted into ceres and I do not see it in the official repositories.

It is my understanding that you do not need this any more.  You might want to try to remove it and see what wants to get updated.  Also, if you have them, you may be able to remove all references to ascii from sources.list .  I have done this to a couple of beowulf installs and it has caused no problems.  However, I do have one beowulf install where I still have ascii references in sources.list file so that I can use the ascii version of network-manager and its openvpn tools.

From what I have read and experienced, there have been at least two major improvements recently that makes beowulf closer to a beta state - first, the polkits got a major facelift a few weeks ago which have been applied with your dist-upgrade to beowulf, and second, you can run

apt install libelogind0

and this will make a noticable change to your system.

And may I add that the change produced with libelogind0 was very nice to see... (-;

I have been setting up bind mounts today for a new system I purchased recently.  I have my notes right in front of me so a simple copy and paste will explain how I did this is ceres, beowulf, ascii, and another distribution. 
I prefer to use bind mounts instead of a separate partition because it reduces the number of partitions on a drive.  This method is pretty simple and quick.  It can also be used to create a bind mount folder for /tmp, if you wish.

- Create the folder that you want to bind to.  Example:

mkdir /mnt/Data/bind-mount-folder/var && /mnt/Data/bind-mount-folder/tmp

- chmod the newly created tmp folder to 0777:

chmod 0777 /mnt/Data/bind-mount-folder/tmp

- Copy contents of the /var folder to the place you designate:     

rsync -avzh /var/* /mnt/Data/bind-mount-folder/var && rsync -avzh /tmp/* /mnt/Data/bind-mount-folder/tmp

- Rename Original folders to save their contents.

mv /tmp /tmp.original && mv /var /var.original

- Create new folders to serve as mount points:

mkdir /tmp && mkdir /var

- Edit fstab with:

/mnt/Data/bind-mount-folder/var     /var		none	bind	0	0
/mnt/Data/bind-mount-folder/tmp     /tmp		none	bind	0	0

I suggest to be sure to put the bind entry in fstab below the mount entry for the destination location.  The second drive location (Data) must be mounted before the "bind mount" location.

- Mount the bind folders:

mount --bind /mnt/Data/bind-mount-folder/var /var && mount --bind /mnt/Data/bind-mount-folder/tmp /tmp

The nicest thing that I like about bind mounts is that the mounted folders and their contents will be included in backup functions like refractasnapshot. 

reboot

This just worked for two linux distributions, devuan and pclinuxos.  If this fails for any reason the system will not be bootable, as /var (and /tmp files) will need to be created.  Be sure that you have a live disk, or another boot method, to rename the original folder(s) and the system will be bootable again.

But like you mentioned, it may be less important today to reduce write functions on a newer solid state drive.

I don't see anything that is not correct, but I can state that at about the time that you were posting this, I was having some problems doing a regular update with similar "File not found" error messages. 

I would try it again later before I tried changing anything.

I think 'apt-get' is being / has been deprecated in favour of 'apt'

Somehow I missed that information.  Thanks for informing me.

Oh come on... please.  Information is "leaked" all the time.  Just accidentally let it slip sometime... (-;

When I marked it for installation in synaptic, it wanted to pull in the *systemd-polkit program, so I backed out.  In my instance, I had in synaptic two programs marked as upgradeable but did not upgrade with a dist-upgrade.  I manually marked to upgrade this policykit-1  program and then the second program.  If I marked the first program in the list, it became marked as broken.  So I marked the second program in the list first (policykit-1) and it updated correctly.  This update pulled in the systemd polkit programs.  No problem, I followed the instructions linked above.  Since slim was installed and in use, I installed the programs below and in installing them, the systemd programs were removed.  I installed:

libpolkit-backend-1-0-elogind (0.105-18+devuan2.4)
libpolkit-gobject-1-0-elogind (0.105-18+devuan2.4)

The information above worked fine.  I now have the shutdown and hibernate options again.  A reboot shows no PID error or anything else that looks out of place.

Edit:  typo/spelling

Try this, as root,

apt-get install devuan-keyring

If you still get the following error,

then look in /etc/apt/sources.list file and make sure that there is only one line containing

If you see only one mention of that line, then look in /etc/apt/sources.list.d/ for that line also.

I think that that line should be:

and should only appear once, preferably in the sources.list file.

For what its worth...
In Ceres,

# cat /etc/devuan_version && cat /etc/debian_version
ascii/ceres
cat: /etc/debian_version: No such file or directory

Have you given Waterfox a trial run?  I don't know about mhtml but the pdf viewing was flawless as well as a couple other things that I have had problems with both in firefox and palemoon.

In case you are interested, here is an unofficial build located on a repository:
https://github.com/hawkeye116477/waterfox-deb

I may be getting a little ahead of myself but, will this work for a ceres install if I change the release name from ascii to ceres in the sources.list file?  For example, can I use this ?