Quite a few points that require comment, correction or clarification -

Your route would require a 4-continent, not 3-continent ticket - Europe, Asia, SW Pacific, N. America.

You may encounter significant difficulties getting 5 D-class seats on some flights, especially longhauls to/from Oz during a peak travel demand period.

At last report, Blands in GIB is out of the RTW business. GIB is not necessarily the cheapest start point in Europe anyway - Sweden has generally been cheaper, depending on currency rates. I'd recommend checking the base fares (and converting to Sterling if applicable) using the drop-down table at the Oneworld Explorer screens at oneworld.com.

You're not allowed to exit your country of origin twice, nor transit the originating airport. However, the ticket is good for a year, so if you started somewhere else, you could return to the UK in January, but still have two or more European segments available for possible travel later in the year.

As mentioned, D class tickets book into A (first class) on 2-class domestic US AA flights, with beneficial impact on tier point earning. (AA flights to/from Canada, Mexico or the Caribbean do not have first class; the front cabin is designated business class on those flights.)