Remember me complaining I was not Keith Packard?
I am still not ;P
But I am kind of closer to a good solution with an up-to-date Xephyr.
You know, at my Faculty of Electronics and Information Technology I in fact learnt just one thing, everything else follows from that: "It works better when turned on" (a small addendum to it is that "computerization of a mess makes a bigger mess"). And this (not being turned on) is the very reason of Wheezy's Xephyr not handling evdev protocol.
So how can it be turned on? Fortunately, it is fairly simple: do apt-get source xserver-xephyr, then apt-get buid-dep xserver-xephyr, and then edit xorg-server-<some numbers>/debian/rules: after --enable-kdrive \ add a new line which reads --enable-kdrive-evdev \.
Then dpkg-buildpackage -uc -us, after some 15 minutes the build stopped with an error regarding libGL; however in xorg-server-1.12.3/debian/xserver-xephyr/usr/bin/Xephyr there was a new Xephyr binary. I tested it and it works ok!
I know that this is a kind of a poor man's solution: the package should build nicely to the end. It is desired to at least know and understand why the build stops and whether it should worry us. Nevertheless, it seems that Xephyr should not be a showstopper for the multiseat config, that is, we are not forced to use a binary from an ancient distribution.