A bold idea, though only partially achievable.
The HVDC line length is comparable to the Pacific Intertie which connects southern California with the Bonneville Power Administration.
There will be a requirement for redundancy in route of the HVDC lines. Incidents such as bushfires underneath the power lines from the Snowy scheme caused blackouts in Melbourne several years ago. No sane energy regulator would allow a dependency on one power line for an entire state. So multiple routes with multiple costs for redundancy and reliability will be needed. These can link up with otherwise isolated grids, such as Esperance or Olympic Dam.

The linkage to the north west integrated system (NWIS) in the Pilbara would also have to contend with cyclones. The Horizon power reliability report of 2006 shows that power losses of 20-100hrs do occur after cyclones. The impact on mining operations, not to mention households, of interruptions of this length are severe and would require additional local generation to offset. In the north west, this would be open cycle gas turbines which are already in use in mine sites, with low costs due to close proximity to gas suppliers. So why build a power line when the load will be met locally.

Given there are existing wind farms in all the air regions mentioned in the proposal, would it be possible to perform a statistical analysis of the independence of these air regions. Taking a years worth of hourly power generation numbers and crunching them to determine how reliable a network of dispersed wind farms can be in the real world would boost your case. Statistical proof that the wind will be blowing somewhere all the time would take the wind out of detractors sails.
There are superb wind farm sites in the south west of WA, with 45%+ capacity factors in real life wind farms. The problem is that there isn't the ability to adsorb the power variations in the grid. The HVDC link across the Nullabour would allow these resources to be exploited fully.

So in conclusion, the Nullabour link to enable increased wind penetration and increased pumped storage capacity makes sense. The northwest link makes much less sense and could be delayed to a second phase of the plan.

Yours,

Lachlan the Accountant.