From a QF customer service rep.
'As per your request, clarifying the rules of the 4 continent Oneworld Explorer fare. LHR and LGW are considered London and as one city, just as JFK and LGA is New York and not considered a ground segment.'
'The new rules that have been introduced in relation to the number of segments permitted is unrelated to e-ticketing. At the moment the maximum number of segments that can be issued on an e-ticket is 16. As you have 20 segments, you will have a paper ticket.'
I needed to do a little bit of messing about to get my itinerary to fit but am still getting over 52k miles so pretty happy with that.
Good luck to those who are trying to re-jig.