Comment 15 for bug 1224098

@Alberto: I did not understand your last suggestion. I had not installed the xubuntu-default-settings, so when I tried to remove that, I got an error message.
I had previously tried removing the XFCE4 and XFCE4-goodies metapackages, then reinstalling them, but that didn't help.

I ended up reloading my original Lubuntu-13.10 Saucy installation from backup, then installed, in this order
nvidia-319
nvidia-settings-319
rebooted
nvidia-prime from the Saucy proposed repository via Synaptic
(updated grub)
rebooted
xfce4
xfce4-goodies*
rebooted

logged into xfce session
chose Use default configuration

It worked.
No error messages loading the xfce desktop. Now I can use either lubuntu sessions or xfce4 sessions, and nvidia-prime seems to be working (at least going by glxspheres64).

I'm not sure why it worked this time - I was 100% sure it would fail. But it worked.

(*I had tried the sudo update-alternatives --config lightdm-gtk-greeter-config-derivatives, but I didn't understand what you meant by that command exactly. I got an error message saying no alternatives available.)