I posted above that I was getting the previously reported 15012 error at the final stage of booking an ex-AMM RTW using the online tool.
I really wanted to get it to work, but finally I have given up and will ticket it with the AA GSA in Cairo.
However, in the process, I had a couple of conversations with the Oneworld IT department and did find some information about the problem which may be of passing interest to some here.

Basically it’s the same old problem that we’ve seen elsewhere in that Oneworld and its airlines are still using two different systems: Sabre and Amadeus. The problem here is that the Oneworld online tool uses Sabre to build and validate the RTW. When you are ready to book it, and if it is to be ticketed by AA (as in the case of first flight on RJ), you are transferred to an Amadeus site for ticketing. (It’s actually www.americanairlines.eu.amadeus.com). This site takes the passenger info, and payment info, but at the end, it looks to Sabre again to confirm the reservation and create the PNR. And it is this link which is causing the problem.

He said the problem most often occurs when AA is ticketing; it rarely occurs with other airlines. (They use a different ticketing site: http://wftc2.e-travel.com) He suggested that I use a different airline for the first flight. However, the only other airlines that fly from AMM are BA - and we know why we don’t want them to ticket it! - and MA which only flies to BUD and then no way to get to Asia without losing more segments.
He said it’s often one particular flight that causes the problem, and codeshare flights are often the culprit. However, I tried eliminating the only codeshares in my itin (the “baby LAN” flights always show as LA xxxx - LAN Airlines operated by Lan Peru) and it still did not work.
I also tried creating a very simple DONE4 with only 6 segments, intending to immediately change to my 16 segment itinerary if it ticketed successfully. But it didn’t.
He even called the AA RTW office for me to see if they would ticket it at the AMM price for me, on the basis that there was a technical problem preventing me from ticketing it online. But not surprisingly, they said they could not do that.
He said that they can send a trouble report to Amadeus for specific itineraries that have failed, but it takes a long time to get a response and in only about 20% of cases do they identify the problem.

End result: they are well aware of the problem, but can’t identify what causes it or how to fix it!

A couple of other things I learned:
1) On the final booking page where you enter your cc info, the total price is displayed; there is a + sign beside this, and if you click that, it expands to a complete breakdown of taxes and fees, which you can print out without ticketing. Quite useful. Unfortunately the Fuel Surcharge is only shown as one item, not segment by segment, but if you go back and change one segment in the itin, you can see what effect that has, if any, on the Fuel Surcharge.

2) As we know, the Oneworld office is in the process of moving from Vancouver to New York. Unfortunately, some or all of the IT staff are not moving (they live in Vancouver; why would they want to go to NY??). The one I spoke to, who has been nursing the online tool since its inception, is leaving next week. Some new staff have already been appointed, but whether that means the tool will be fixed quickly, or take even longer to fix, remains to be seen.

Happy Travels