Quiz results being sent to the LMS again when a learner goes back into a completed course

Disclaimer: I have enough knowledge of basic LMS architecture and operations to be dangerous, but not enough to do more than tinker and test.

I would say that it is not generally possible for a learner to go back to a completed course without the course file pushing the results once more to the LMS. Depending on which LMS you are working in, there may be a way to configure the user profile to allow returning without sending results, but that would be a course configuration within the LMS, not Storyline.