[racket] instantiating multiple sandboxes with gui's

I want to report back to the list that Robby and I went on to exchange
quite a few more emails.

I'd gotten overwhelmed trying to juggle too many concepts and moving
parts. In addition to Robby's explanations, I had source code to
racket/sandbox, dynamic-rerequire, enter!,
mred/private/common/wx/queue.rkt rattling around my head. As well as
pages of docs about namespaces and modules and eventspaces. Buffer
overflow.

I thought I could strip it down to a simple working case, then build
up... but induction wasn't the way to tackle this. Nor was poorly
reinventing racket/sandbox, even though I learned a lot in the
attempt.

Robby was very patient and extremely helpful. I appreciate it hugely.