Broken functionality after update (gnome, pam, glib2, systemd, etc.)

Question

I upgraded my system yesterday after going for a month or two without updates and a ton of authentication-related stuff broke. Here are some of the issues/symptoms I'm currently experiencing:
Seeing this message in journalctl:
May 22 13:13:50 deepspace1 slim[203]: pam_loginuid(slim:session): set_loginuid failed
Pulseaudio is not started when logged into GNOME; aplay does not list any cards when invoked by my user:
[mcmlxxxvi@deepspace1 ~]$ aplay -l
aplay: device_list:268: no soundcards found...
[mcmlxxxvi@deepspace1 ~]$ sudo aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: NVidia [HDA NVidia], device 0: ALC662 rev1 Analog [ALC662 rev1 Analog]
Subdevices: 1/1
Subdevice #0: subdevice #0
When I start pulseaudio manually, it only lists the dummy output.
USB sticks no longer auto-mount
I had been using GNOME 3 on forced fallback and logging in through SLiM.
I tried to upgrade libreoffice, which made me upgrade glib2 (to 2.36.2-1), which caused some other problems (pnglib, etc.) so eventually I upgraded everything (including kernel, systemd, pam (to 1.1.6-3) and gnome (to 3.8)) without rebooting. A couple of hours later I was suddenly thrown out of my session and saw something about some journaling/logging utility being passed too large a block (cannot find it in the logs at the moment) on the console. I rebooted and after logging in, gnome-session-failed (the "Oh no" screen) was started. GNOME started successfully when I logged in as root.
Googling around, some threads on here and based on the PAM message above, I tried to downgrade glib2 (to 2.36.1-3, 2.34.3-1, 2.34.2-2 and 2.32.4-1), to upgrade pam (to 1.1.6-4, which had appeared in the meantime) and then downgrade it (back to 1.1.6-3 and then to 1.1.6-1).
glib2 downgrade led to nowhere - later versions led to gnome-session-failed, while with earlier ones GNOME would not start at all). Downgrading pam led me to a situation where I received a "Login incorrect" after entering the username (both console & SLiM) which I had to fix through init=/bin/bash .
Following the advice here I cleared the hidden GNOME files and was finally able to log in (default mode, not fallback). Enabling fallback via gsettings (gsettings set org.gnome.desktop.session session-name gnome) brought back gnome-session-failed.
So I'm led to believe that I originally had 2 problems: fallback mode, which I read had been disabled, and the authentication problem. I'm currently struggling with the latter and would like help on how to approach the situation.
I'm not sure which logs would be needed for troubleshooting, and I'm still quite clumsy with journalctl, but will provide any logs requested.

Answer 1

You should *never* do partial upgrades.  There is a reason why everywhere you see people saying that "partial upgrades are not supported". 
I am not sure how you can be "clumsy with journalctl".  As far as its most basic functionality, it is pretty darn straight forward.  Though that is assuming you are willing to do a quick read of the man page... or even the output of "--help".