Hi U
Good points.
However, I think all those more complex issues are trite. That is, they are all well known.
Every now and then, lots of the holes in the pieces of Swiss cheese are going to line up. Fortunately, the chances of them all lining up are extraordinarily remote and, as in this case, no collision occurred. Whether that outcome in this case was a matter of good luck rather than good management is, I reckon, moot. We don't know whether decisions and actions other than those that were made and implemented would have resulted in the aircraft remaining further apart than they ended up being.
That doesn't mean that nothing should be done to mitigate even those risks. However, when you think through what the mitigation options would be, and how much they would cost, and that they would still be no guarantee of collision risk-free skies, you get back to reiterating the basic requirements of the rules and airmanship that have been around for around 100 years. ('Affordable safety' is a fact of everyday life, not a radical theory. That's why not all roads are divided, there aren't traffic lights at every intersection and there's only a millimetre of insulation between you and a voltage that will kill you.) It may get to the point that the volumes of traffic at YMIA are such as to justify further mitigations - CAGRO, Class D, revival of the AFIZ concept - but they are no guarantee.
I'd be interested in others' views on whether my decision - if I were in XGA - to go around and track to parallel runway 09 to the south as soon I became aware of the aircraft on base, and announce that on CTAF - is a good one or a not-so-good one. (I fly into and out of YMIA occasionally.)