delta3,
Nice summary of the problem, but I would characterize the issue as less of "rule of thumb" and more of "described model" where a rule of thumb is a calculation simplification (6 knots per thousand for Vne, or 2 degrees per thousand for OAT lapse), and the model is like "what happens in VRS or IGE."
I am reminded of the scene in the movie "The Sand Pebbles" where Steve McQueen describes the ship's steam plant to an astute chinese engineer, using a dragon as the steam and saying the the main stop steam valve kills the dragon. The explanation model we chose is determined by the sophistication of the student and the communication barrier that exists (language, or technical background, for examples).
I do not advocate models that state pure mathematics (except as proof of the proposed model) because the math is not an intuitive model for most people. I also do not believe the use of incorrect physical explanations is advisable ("IGE goes OGE when the bubble is too far away", or "VRS involves rotor stall") because this is simply wrong, and potentially misleading.
I think the model we use should have reasonable correlation to the actual case. I am reminded of one of my favorite quotes, “If you can't make it clear, you don't understand it yourself.” A true master explains in a valid way, without confusing his student. Think of Einstein's classic book "On Relativity." I think we owe it to our students to understand the condition well enough to explain it without inventing new physics.