Can’t bounce back and forth between 91 and 135 during the same duty period assignment . you may have a 91 flught assignment, but you are still held to your restrictions for the 135 duty period assignment.
That is an incorrect statement! You can bounce to Part 91 at completion of your Part 135 and have no duty limitations. You are still on-duty, but not held to the Part 135 14-hr duty limitation (since you are Part 91). It cannot be counted as part of your upcoming rest period until you are actually free from all restraint.
Stop trying to tell pilots in a different class and craft they must plan to return to the base within 14 hrs. That is totally incorrect, here and where you currently work!
Or, post the FAA letter of interpretation you got (I mean, that you didn't get).
Yes, its the planning: and you can PLAN to exceed a 14 hour duty day by PLANNING ot conduct a tail-end reposition under Part 91 sans medical crewmember and patient. Its how professionals operate!