I had a wild idea, possible a result of too much ignorance.  Run the main debugger in the user process, with a separate root and event loop.  Add to the communication protocol options to send a list of breakpoints and modules to be traced into and to send (module, lineno) pairs back, for highlighting in the editor.

I am willing for a revised debugger to not work with -n.  We want that to go away anyway.  Roger Serwy sent some of us a prototype pipe implementation a year ago when I was busy with GSOC, and I did not pursue it yet.