You're right if you think you can be asked to fly or do anything else beyond 14 hours (10 hours off in last 24 is the technical definition, which many just can't seem to grasp) in Part 91 flying. What some just can't seem to grasp is, that because of a lot of stupid screw ups in the past, HAA has been singled out by the FAA such that if any med crew is aboard, even deadheading, it's now Part 135 for the HAA operation.
However even in HAA, you can leave them and fly the aircraft back with no other occupants on board.
You ALWAYS retain the option as PIC to tell your employer, I'm too tired. Then they have to deal with the consequences. And you may have to deal with them, as well. That's the way it goes in the Big Leagues. If a Guy wants a Kindergarten atmosphere, my advice would be to go into early childhood education instead of professional flying.
There is one little hitch in all this, namely, what your Company will and will not allow you to do. If you work for an operator that says, at 14 hours (10 hours off in the last 24, again, technically...) you are done, period, no more flying for you of any type or description, well, that's it. You land the aircraft where you are at less than 14 hours and either get a hotel or your relief pilot drives up and swaps out.
And THAT is THAT.