I just wanted to add my condolences. I was once part of a small group that collected the wreckage of a glider that had been in a mid-air. It was a sobering experience.
I also wanted to make an observation from the point of view of someone who has a background in psychology, though not in cognitive psychology, which is the relevant field here. Anyone who puts their trust in the human cognitive system should look at the articles on Wikipedia that discuss *change blindness* and *inattentional blindness* (can't post the links). And they could do worse than look at some videos by Dan Simons and Chris Chabris, which are easy to find on YouTube (once again, sorry I can't post the links).
It is important to recognise that, even if something is right in the middle of your field of view, your cognitive system may not pass that information to your consciousness. I am not taking sides in the debate about TCAS or anything else, just saying that every system has limitations, but I suspect that not everyone is aware of the extraordinary limitations of human equipment. Perhaps a short course showing those limitations ought to be part of pilot training, if it isn't already. The same goes for driving.