Well, lots of dumb mistakes as I suspected.
Has to be x-action, not action.
So, for completeness this is what works:webbrowser.open("workflow://x-callback-url/run-workflow?name=Play%20Specific&x-success=pythonista:")
And this returns right away because this workflow launches music without waiting for a return. So, from workflow's standpoint the workflow finishes immediately.
Closed and Solved