Apparently the ROS topics have an ambiguous behavior. I have also tested publishing through the rostopic pub and it showed a parallel behavior. But when the publication came from another node it apeears to wait for the execution of the last callback on a sequential approach.
OMPL does have Python bindings. There is a section on the site documentation explaining the differences between the Python and C++ APIs and some samples. That may provide you with a starting points, though I don't see any full tutorials using Python as of this writing (alternate long-lived link).