Sounds like a lot of flying, you'll certainly have no status problems this year!

Two notes:

1) First the bad news. I was under the impression that the transpacific routes did not have Economy Plus Seating. This may only be 777 flights, but maybe somebody can comment on the availability of these seats over the Pacific.

2) The good news is that as a General Member, I never had a problem getting E+ seating. The only restriction is that you can't pre-reserve Economy PLus seating, but if a seat is available at checkin, you have every right to it.

I would imagine that the agent at MEL should be able to assign E+ seats for your entire outbound journey (if available), and the LHR agent should be able to book seats all the way back.

Have a fun trip, either way!