Steve,I tried your trick (--force_confmiss) on my totally broken unstable. It indeed added the /etc/X11/Xsession file back again, I managed to do startx as root. Not as a normal user. No trace no nothing. X starts and then fails. It is not only KDE : I changed my own xsession to fvwm without any more luck.
=> breakage is more important than just the missing files