Disable User List on GNOME Lock Screen

The current user label is not the same thing as the user list.

However, when locking the screen (instead of logging out, which ends the session), the login field shows the user.

Yes, the lock screen purposefully indicates that someone's got an active session and they might return soon. (Or, in single-user devices – whom to return the device to.)

The login screen shown after locking a session is apparently not governed by gdm, but by gnome-shell (I can't verify this atm, I'm sure I read it in an answer but I can't seem to find it).

Yes, that's the usual design. The lock screen is nearly always provided by the user's own environment.

On Linux only a single process "owns" each graphical virtual console at a time – if GDM runs on tty1 and your session on tty2, then GDM has no way to block input or suppress gnome-shell's output on tty2. It only has control over its own VC.

Sure, the user's environment may switch to the login screen on tty1 (as some DEs do with LightDM, and is also possible with GDM), but nothing prevents the user from switching back to tty2, which then still needs to be displaying something (and hopefully not an unlocked desktop). So the desktop might as well show a lock screen directly.

In fact, in the current implementation even the main GDM login screen is really just gnome-shell in a special mode (running on a dedicated gdm account). This means all GUI code is actually shared between the login screen and the lock screen, so try deploying the same GSettings parameter to all users (using the dconf profile feature).