Thanks for all your answers, I think I came to a better understanding of how the ONE tickets work now. There are still a few things on which I remain unclear though:

Why is it so that the One world trip planner does not find flights from, say, Nairobi to Buenos Aires ? Does it not automatically find potential connections (in this case, lhr/mad/bcn) ? Does it not find connections at all? If such were the case, how to plan with it then?

Please, let me know if this itinerary seems possible to you with a LONE5 or 6:

CDG-LHR-JNB//NBO-LHR-EZE//EZE-MEX//MEX-NRT//SIN-LHR-CDG

Also, how do we know whether a flight with one or two connections constitutes one, two or more segments, since the oneworld trip planner does not allow to plan for all stopovers/connections ?