Plan to drop the medcrew somewhere and you can PLAN TO operate Part 91 well beyond 14 hrs (since you are no longer Part 135).
Not talking about once I accept the flight... I'm talking about the plan TO accept the flight. Plan to fly A to B to C under Part 135 within 14 hrs. Drop the crew/patient and plan to fly C back to A (or anywhere for that matter) after 14hrs.
YOU DO NOT HAVE TO PLAN A FLIGHT BACK TO THE STARTING POINT WITHIN 14 HRS!