If you want to maximize the xONE ticket a good strategy is to use the 20 coupons for long flights and buy separate round trips. So instead of adding SFO-SEA into the RTW just buy the ticket separate.
Use the coupons for things like LHR-Dubai or Muscat in Europe (Oman is really nice), or NRT-SIN in Asia or in the US Flights to Alaska or LAX-Puerto Rico etc.

Also my recommendation is get a business class ticket and use good airlines. There is a lot of discussion here and different preferences, but in my experience CX and JL are very good, IB, QF and BA (don't use from US to UK - no miles on the AA program) are good, AY is ok, LA depends very much on the seat type, avoid AA.

Then also go over to the Star Alliance board and read about their mileage based RTWs. For some itinaries they are better solutions, NZ and SA have both an excellent business class - which could cover LAX-LHR-CPT-and on to Asia, then NZ back to the US. In the US it would be united. Only in Australia you would need to buy some extra sectors, but they are inexpensive.