Good news!

The Xephyr in xserver-xephyr 1.12.4-3 now contains a patch restoring correct command line parsing, so now only --enable-kdrive-evdev is required in the control file.

Another upgrade

This time it is lightdm: the latest version in wheezy is 1.2.2-4.
The same procedure is still valid: apply the two patches, rebuild and voila.