I'm just going by that article, not experience.
Something I've learned many moons ago was to use an option in the kill command, not sure how it came about but I always use: kill -9 PID, PID being the process.
Have you checked in /etc to see if any of those files exist?
I have a smooth operating Debian and those lock files do not exist, (passwd.lock, group.lock and gshadow.lock). If one or all those files exist, then this is more than likely what you need to do.
I checked and I also have those two processes, useradd and groupadd, they don't show up when I do a: ps -aux, but do show with the ps -ef.
Also, for all three of those files, I have in /etc/ three versions, example:
It is also possible the "cannot lock /etc/gshadow" error arrises due to the file not existing, if so, might be worth trying to create a blank file by that name with command: touch /etc/gshadow