Looks like the problem is that the twisted.internet.reactor is imported inside a block guarded by
if __name__ == '__main__'
When you import working_code that block doesn't run, because of the guard, so terrific_method tries to access "reactor" which doesn't exist. I guess your question is really how to get access to the reactor in working_code.py, since you can't import it in the top level namespace because you have ti install the qt5reactor before importing twisted.internet.reactor (right?).
I don't know if there's a generally advised way of handling this, but I would simply put all the stuff in the guarded block in its own file and make sure to install the qt5reactor before importing anything else.