You point in highlighting this was that a pilot (certificate holder) MUST FIRST plan the flight all the way back to the base within the 14 hrs.
AGAIN, that is not the issue here!
A pilot can PLAN to take a patient to a facility away from the base, drop the patient and medical crew off at that facility away from the base within 14-hrs of his duty start time, AND THEN fly Part 91 back to the base under Part 91 well after the 14-hr restriction. His FLIGHT ASSIGNMENT is planned to end at the away facility, thus, he is no longer an HAA Operation.
Similar scenario: Take the medical crew to the patient at an away facility and drop them before the 14-hr of duty, then fly back while they prepare and stablizie the patient to be flown (which in many PICU/NICU situations, takes forever), and have the on-coming pilot go get them later! You can PLAN to exceed the 14 hours total duty time provided you are FINISHED with the HAAO before 14-hrs and Part 91 reposition after the 14 hours! All done legally and without having to lie about "circumstance beyond the control of the certificate holder" since planning for an unknown is actually a forseen situation under the control of the certificate holder during the planning phase!
Same is true of an "average time" situation, which is based on the mean, which means it is realistic to expect half the times to be above the average, and if you plan using that average time, you are planning to exceed the planning at least half the time! Which is well within the control of the certificate holder for not being very realistic and fails the spirit and letter of the Sloan (2015) letter, and all its citations!
So, again, you are trying real hard to evade the fundamental issue here and use it to turn down flights because you believe the initial planning MUST return you to your base before 14-hour duty day limit of Part 135.267 expires. Nope!