OK, I tried to reproduce the Pacman troubleshooting - Manjaro instruction from step 1 and on step 2 got the same issue as OP have.
Now 2 people has the issue 
I think that the instruction has the issue
How we can install any new package, for example
sudo pacman -Sy gnupg archlinux-keyring manjaro-keyring
If we removed all local keys by
sudo rm -r /etc/pacman.d/gnupg
than we made a package manager to be unable to validate any further packages.
@saintjules , I was able to resurrect keyring issue by skipping the second step and to get some core keys import via direct connection from Internet during step 4.
Try next:
1st step.
3rd-6th steps (note that 4th step could be really long (several minutes)).
But after that re-do action in the instruction again, but start from step 2 and redo the next 3-6 steps again.
Perhaps my suggestion is overkill in some points (it is the subject to review by others), but it works and I shared the working temporary issue fix fast.
Please let us know how went your keyring issue fix, were you able to update OS after it
sudo pacman -Syyu