So I decided to persist with the Frankfurt reservations team. I wrote, copying the rules and asking them to highlight for me the section where it says only 4 transits are allowed in the USA. I get a reply 12 hours later saying to see the highlighted section. And they highlighted this:

MAXIMUM OF 4 INTERNATIONAL ARRIVAL AND DEPARTURES
IN THE ONE COUNTRY

Which is of course irrelevant to their original stipulation that only 4 transits were allowed in the USA, and my itinerary certainly doesn't violate as I arrive any country, USA included, once and depart once.

So I have written to them again pointing this out. it's becoming comical at this point. But every message comes from someone new, so my hope is that I eventually hit someone that has a clue about round the world tickets.