I had this same experience booking a DONE4 ex-AMM. But I did eventually tinker with the itinerary enough until the online tool finally accepted it.

It was far cheaper to pay the re-ticketing fee to change it back to what I wanted originally than it was to pay what the RTW desk was quoting.

That said, is the MEL-LHR-AMM at the end legal if you already did DXB-LHR-LAX at the beginning?