Keith - if you could draw that you would be my saviour! I've tried to sketch what I think you are saying but am getting a bit muddled.
Alternatively is there a reference book to explain this?
By the way I am a PPL and was asked this question from a friend of mine with an interest in aviation. I couldn't answer why Mach number replaced IAS at high altitude given the IAS will always give you a reading of some fashion changing with altitude and that IAS will have an equivalent Mach Number at any altitude (is this true?)
I can tell you even a jet pilot I know couldn't answer though that was maybe because he didn't know how to explain it properly.
Its a bit sad the things that keep you awake at night!