Upvote:2
I am not entirely sure what your question is, but I think the answer is that you need to use an arsenal of tools, particularly http://matrix.itasoftware.com (but note that no tool is comprehensive due to the enormous complexity of multi city journeys). I have found Google Flights of some use as well, and ExpertFlyer ($100 / year) is essential for me to evaluate fare routing rules and booking code override tables, which the Matrix doesn't reveal. You also need to acquire some "feeling" or I should say, proper understanding, of how fares actually work
If you are a oneworld fan, AA.com is quite good at booking code shares and prime codes on their partners (I am sitting on a BA-numbered flight booked on AA 001-numbered ticket stock right now, just waiting to depart Bergen!). Delta.com used to be good for intra-European sectors on Air France / KLM because you could get an identical itinerary (even with no Delta flights) minus the American Express fee. I would presume therefore it can handle multiparty journeys at least between Europe and the Americas.
Once I have an itinerary I am happy with on the Matrix, if I cannot set it up myself on BA.com or AA.com, I usually call BA and ask them to ticket it by phone, or I'll print it out and drop it on the ticket desk next time I'm at an airport. (Actually, I also have a contact at BA who will setup itineraries for me by email, but I am not willing to share who he is publicly.) But my point is that to do anything complicated, you need to talk to a person.
The websites of every carrier (and travel agent) are extremely restricted in what they will sell you.
http://RTW.oneworld.com does actually work quite well provided you know the rules on the oneworld explorer fare (which is an enormous topic).
Please note that round the world fares continue to be priced by hand by dedicated back office staff and therefore, with the exception of the special alliance tools (which only provide an estimate) you will not see RTW fares on booking sites. I am aware Amadeus is looking to bring full auto pricing to RTW fares very shortly.