It worked! Thank you so much!! Had to change it to alpm:x:973:973:Arch Linux Package Management:/:/usr/bin/nologin to dodge existing entries, but pacman -Syyu is now running!
It’s wild to me that of this whole massive blunder, the login issue seemed to involve /etc/passwd but was actually /etc/shells, and the pacman issue seemed to involve /etc/pacman.conf but was actually /etc/passwd 
Is there any post-issue cleanup you’d recommend?