So, I could actually show you the emails where I helped move JSON from a MAY implement (with XML as a SHALL), to it being a proper first class member of the spec. The FHIR core team was and continues to be interested in feedback from experienced integrators. I read your comments here occasionally; you're smart, participate!

You use the phrase 'lingua franca' which provides and interesting analogy - it was implemented globally by a group of imperialists who controlled the trade transshipment points of a big chunk of the world by force of arms; is there a corollary in health care?

Actors on the web that have a profit motive to play together have adopted a set of norms (REST, JSON) to manage tasks that are usually trivial compared to medical knowledge representation. We still can't get an iCal right, for goodness' sake. Everyone else is buttoned up, and FB and GOOG don't exactly share network data with each other.

Regarding point four: if it get's specific to times, then you do have to indicate time zone, just ate the dateTime level: "If hours and minutes are specified, a time zone SHALL be populated." (https://www.hl7.org/fhir/datatypes.html#dateTime)