The input method doesn’t work inside Xephyr? Sometime you find out that .xsession-errors says

 Preparation of font set "fixed" for XIM failed.

The font path could be wrong. Make sure that the path to misc is the first one in row.

 xset +fp /usr/share/fonts/misc

would fix it for the moment (path might vary).

Compiling Xephyr I missed the following switches:

./configure --enable-xephyr --enable-kdrive-evdev --enable-kdrive-mouse --enable-kdrive-kbd --enable-kdrive --prefix=/usr --with-fontrootdir=/usr/share/fonts --libdir=/usr/lib64

Thanks for paying attention!