Sorry just got back...
remember XFWM4 has many hooks to xfdesktop.
'MDM when you configure tries to call call the Desktop manager, but doesn't recognise xfdesktop But
in LMDX sees nautilus so uses (nautilus causes problems from here on). In LMint 13 XFCE RC just shuts dowm XFWM4 because no nautilus.
A simple fix may just be saving session (in setting>startup and session) and making sure the save session is always unchecked on logout.
P.S. remember to be able to deleate MDM compleatly it is in as a 'sys service' also (I use Bootup-maager to stop service) where you'll be directly dropped to a CLI. Resart X (I use 'startxfce4' then full deleate MDM (I use synaptic) Then reboot again and install GDM.
Haven't got around to fix spell-check yet