Actually woody, your argument supports fixed boundaries for CTAFs/Areas. The only problem with your scenario was that 5nm was the limit. If it was that airspace actually used by a jet, then the situation would probably have been completely different;
before the Jabiru got to the boundary (say 10nm), it would have made a call, alerting the 737 before windscreens got filled. Newcastle was always going to be a dog's breakfast, as was pointed out to the proponent of the Class G Trial and then E there.
Your example highlights two things: the inappropriateness of low-level Class E airspace into a CTAF area for jet operations in busy airspace, and that 5nm was never appropriate/too close in some circumstances; the only reason we got lumped with it was because "that's the way they do it in America".
What is needed is clear, simple rules so that even the most lowly-houred troop can understand and comply with. "Approximately 10nm" doesn't work in many situations where jets are involved, not that the most vocal Free in Gers understand.
Regarding your verbal diarrhoea student, there's only one person to blame for that! Unfortunately, as per your example, it takes years to turn the wheel around.