"Only one stopover in Europe is allowed"? What? Says who?

The only restriction on stopovers in a OneworldExplorer is in the continent of origin (Africa in your case) in which you are restricted to two.

You are wasting a lot of time in conversation with someone who does not know the rules. Moreover, you say (wrongly) that you can only have one stopover in Europe and then show an itinerary with two (DUS and MCT) - you clearly haven't come close to understanding the rules. Can I suggest you talk to someone competent (such as BA's US RTW desk). The number of people working for the OneWorld airlines who do not understand OWE rules FAR exceeds the number who do.

I repeat that I have no problem with your routing within the US, so you don't need to justify that at length.