Love the forum and looking for some insight.

I have booked One World Miles Saver Awards (OW130C) for both me and Mrs. BKK747 to use in conjunction with a cruise. I have been informed by AA that I can take 16 segments max, but that we must use AA and two partner carriers to take advantage of the award.
The current itinerary is Miami-LAX via AA, LAX-Brisbane via Qantas, Rome-London via BA, and London-Chicago via AA.
I’d like to avoid the London connection and go Rome-ORD non-stop, but meeting a roadblock from AA in the booking without the third carrier.
Any ideas would be appreciated.