andyclift,
The problem with your wife's setup is that the Zumo 550 doesn't actually output music as bluetooth "music". It actually outputs it as "phone sounds".
The upshot of this is that your wife's Scala always thinks she is on the phone. The other drawback is that the quality of the music will be terrible as it's only telephone quality.
There are 2 options for you, both which will result in you being able to talk to the wife and also her receiving proper stereo quality music. The options are either connect the Zumo to the Scala by a cable (what my wife does with her Zumo 550) or connect an A2DP transmitter to the Zumo and pair the G4 with the transmitter. If you do this then you'll have to go into the Zumo settings and set the bluetooth to "phone only". This will mean that the Zumo talks bluetooth to the G4 for phone calls, but sends music and navigation out over the wire to the transmitter. This will get your priorities correct on the G4 and allow everything to work.
One point to note - if you connect music to the G4 via a cable then you will lose the VOX feature and have to press the button at the beginning and end of the conversation to open and close the intercom.
Dae.