I tried the oneworld booking tool for a DONE4 originating CPT. There is a red triangle on the first segment warning of the transit without stopover on one of the European transits. I loaded in
CPT-LHR-DXB-LHR-LAX-SJU-DFW-ANC-DFW-JFK-HKG-NRT-CGK-NRT-BKK-LHR-DAR.

I was not asked to choose flights, don't see anywhere to do so before completing itin. It says my itin is unfinished, and when I push the finish button, it takes me to CPT from DAR and says I have too many segments. I'd love to omit DAR and just go to CPT, but I know that's against the rules as I recall them. Obviously, my last London transit will be the short stop, and I don't see any other problems. Any hints?