Will Mir display server be able to launch X11 applications?
The Mir server will not be able to communicate with X applications but you will be able to run the Xorg server with a Mir backend that bridges X applications to Mir. This is the same solution as done in Apple (XQuartz), Wayland (XWayland) etc. The server will be called XMir.
I was searching the wrong words, this feature is planned for October 2013.
The most prominent toolkit choices which include Qt/QML, GTK3, XUL etc. will get native support, others - an in-session rootless X server that is integrated with Mir.