I installed lightdm along with the gtk3 greeter.

Upon trying to start cinnamon it failed, however it did work in *Software 
Rendering* mode.

Gedit still opened from the menu.

Running cinnamon-settings seems to have worked.

python-pam is installed for me as well, the 2.x version NOT the 3.4 version.
pam, pam-base, pam-libs as well.

Perhaps try to reinstall the pam packages?  I'm sorry, but I really have no 
idea what else it could be and I can't seem to replicate your issue on my 
machine.  My machine is also far from fresh.

If you can replicate the issue on a fresh install, I'd be happy to try it 
myself.  I should be getting a Lenovo T420 in a couple days, I'll probably 
want to install void anyway.  If I remember I'll try to test it out then as 
well.