I struggle to see how Woodside could justify a significant...

The policy was implemented at a time of significant investment into the offshore sector and it was WA's way of ensuring the people got a benefit back from their resource. Given that the offshore fields are pre-dominantly located in federal waters they could only have so much of an impact on the reserve requirements without hitting significant backlash. Hell they received significant backlash for the 15%.
Onshore however is directly under the control of the state government, requires far less capital to bring into production and was and still is only a small contributor to the overall domestic gas market.

I struggle to see how Woodside could justify a significant investment in Pluto train 2 for Scarborough whilst they were well aware of the shortfalls already looming for both KGP and Pluto before the proposal even begun. I am only speculating but it must be part of their long term plan that will justify bringing Browse online otherwise it will likely be left as a stranded asset. This way they would have reasonable justification that it is the only sensible solution to avoid shutting down trains at a liquefaction plant that is operating properly. It will be interesting to see how this landscape unfolds as Barossa and now Scarborough face cultural claims from indigenous parties which has been instigated by activist groups using government funds. It is an odd world we live in no doubt.