I would indeed not aim for compatability with the current stars client, unless it is trivial to convert the move / order files to some sort of nice format.

Multiple concurrent plays could actually be done as completely separate processes, so the global lock does not come into account. That is how web servers like nginx do it automatically, so that should be no problem. I am envisioning nginx+flask or something like that for the API, and if needed celery for the turn processing.

For the record, I currently use python 2.x and write in emacs, but probably 3.x is fine here.