Not being anywhere close to Milwaukee, I can only address the first question because it was explained to me by a local some time ago and is a common practice in large metro areas to keep radio traffic to a minimum. The dispatch is put out as an address only because the station being dispatched is given all data in a paper text message which is handed to the Batallion Chief or the driver of the recue unit just before they respond. The address-alone broadcast warns any other units already out mobile in that district that a response is under way and to be cautious of traffic in intersections so collisions between equipment is avoided. This is what I was told, but I have not verified that info independently.
I'm sure that local Milwaukee members here have a full list of numerics for the various divisions and units - I'd be interested in that as well just in case the question comes up again.