Funny thing is when two professional Pilots are self separating in busy CTAFs this type of crap does not happen.
Suddenly put ATC in the middle and it complicates everything. Combine that with inexperience (like Darwin ATC) and it compounds. I had Darwin ATC once tell me that I should have the traffic in my 3oclock on TCAS (flying metro at time )
I'm not saying CTAFs are perfect, far from it. However when you are talking direct to the other Pilot, you know what each other are doing.
When you put ATC in the middle, often you don't know about each other or what each other are doing.
Holes in the cheese indeed.
Have you read the ATSB report? Have you thought that maybe the 737 and WW were not the only two aircraft in the sky? In this case, ATC were "getting in the middle" of something like 16x F18s, 1x A320, 4x Hawk, 1x WW24 and 2x737, plus an active circuit at WLM with two or three. I'd love to see that work CTAF. YWLM don't operate on discrete Approach High/Approach Low frequencies unless there is considerable traffic volume, including military fast-movers.