There are several rules/scenarios in this business that will generate pages of interpretations just like this one. It's maddening when our "guidance" is so vague and convoluted that a room full of professional commercial pilots with decades of experience can read the same paragraph of regulation and come away with completely different understandings.
Why oh why won't the FAA just simply answer it directly. Say it. Out loud. "A HEMS pilot will be on the ground at 14 hours duty time...wherever you are. No exceptions." Define when duty day starts and define when duty day ends, specifically for HEMS pilots (since we seem to be the only ones who have the problem and we now have our own subpart L). Easy peasy. Now, I don't care if you guys decide on 12, 14, 16, or more, and I don't care how you want to manage the passengers during that time period, but say it, dammit.
The company I work for (one of the bigs) made it easy for about a minute a couple years ago and said that. 14 period. Then, "upon reflection...there are exceptions yada, yada $$$ use your best judgement." Back into the gray. During your enforcement action, the FAA will clarify it for you. ;)