... if you need an RFDS aircraft anywhere north of a line YPLM - YBAS, surely the jet would be better off coming from Broome rather than Jandakot.
It doesn't work that way.
If you have an accident somewhere other than one of the major centres (Broome, Derby, Wyndham or Kununurra) then it won't be the jet coming to get you. It will be the PC-12. If you then need to go to Perth or Darwin the PC-12 will fly you to one of the major centres, from where you will be transferred to the jet.
It takes hours for that PC-12 transfer to complete. During that time, the jet can be making its way up from Perth. In the end, it will have taken exactly the same amount of time to get you to your final destination. And if you had your accident in one of those major centres, you wouldn't be immediately transferred, anyway. You would go into the hospital at that centre for stabilisation - again taking hours.
If all the communities had airstrips that could take the jet, then it would be a different story. It appears only Balgo Hill has a strip capable of taking the jet - assuming the RFDS are intending to operate on gravel.
What about Halls Creek and Fitzroy Crossing? Both are sealed and are jet capable, but again, both of these places have their own hospitals. And the runways at these two places are a little on the short side, so jet operations may not be possible all year round. This also applies to Balgo Hill (the combination of high elevation and gravel would put the required takeoff distance outside of this jet's capabilities during the hot months, I imagine).
... cover Christmas and Cocos Islands direct.
Christmas probably, Cocos definitely not.
So, what is the advantage of basing the jet in Broome? What is the medical scenario that requires the jet speeds?