I thought there was an X server for Plan 9 (edit: yes; https://plan9.io/wiki/plan9/x11_installation/index.html ), but it kinda defeats the point unless you use something like https://github.com/gerstner-hub/xwmfs on it. CDE... something like CDE probably, but actual CDE would be painful and really defeat the point unless you ported its IPC to a Plan 9 native version.

You could, but if you want Unix, you already know where to find it.

Not trivially. There's a lot in how Plan 9 is put together that makes this a monumental effort.

(That said, there may have been an X server brought in at some point, but don't quote me on that. That's the least of anyone's problems in undertaking a CDE port though.)