I think you are on to something. In the absense of any further contact to date by either Volvo dealership (obviously making too much money at present) I've continued researching on Google myself.
The SAE has this paper online which I think, in conjunction with your findings, probably answers the question and confirms what you and others have said regarding the expansion valve.
http://papers.sae.org/2015-01-2275/
From the abstract:
"In the thermal expansion valve (TXV) refrigerant system, transient high-pitched whistle around 6.18 kHz is often perceived following air-conditioning (A/C) compressor engagements when driving at higher vehicle speed or during vehicle acceleration, especially when system equipped with the high-efficiency compressor or variable displacement compressor"
Yup, mine is high-pitched and 'provoked' into being louder by accelerating. The compressors Volvo use are of the variable displacement type.
"the whistle is generated from the flow/acoustic excitation of the turbulent flow past the shallow cavity, reinforced by the acoustic/structural coupling between the tube structural and the transverse acoustic modes, and then transmitted to evaporator"
So when the A/C is off, no refridgerant flow = no noise. The noise is being transmitted by the physical elements (refridgerant lines) to the evaporator coil in the climate unit, so that's why it is heard from behind the dash and from the vents.
I suggest that your cable-tie and packing works to damp out some of the vibration in the lines, quieting it down.
In short - thanks for taking the time to experiment and feed back. I think I will instruct my usual garage to forget diagnostics and just replace the expansion valve and re-charge, not mess around with the compressor.
That said, whilst it looks as if the cost of a new expansion valve is about a tenth of that of an exchange compressor, what's the betting that the dash has to come out and labour costs are massive..