Originally yes, I first tried to replace and after that failed, I renamed the library to allow the installation of both with and without xcb. Are there any changes to Gtk and/or Hildon (and if yes - in which repo) that do suggest that they work with an xcb-enabled libx11 now?

IIRC the problems with libx11 with xcb enabled concerned the hardware keyboard and not the virtual one. I remember trying to get back the old library with just the virtual keyboard (which has no return key) on the terminal emulator. Luckily I had sshd installed, too.