saab 9000 coolant loss

Nah, get a new matrix installed, you will need sooner or later. Radiator seal might clog up the cooling pathways.

Could be both the radiator and the heater core. If it's just the latter, in my experience there's no sign of wetted ground underneath the car at all.

There are two rubber flaps on the firewall. The upper, and larger one, is a drain for the area below the windscreen, as you said.

The lower, and smaller one, is the evaporator drain. It's where the condensate from the A/C unit should discharge. Since the A/C evap and the heater core are essentially in the same housing, it would be logical to conclude that a leaky heater core would result in coolant coming out of that drain also. I'd go with the core replacement and fix it right; the stop-leak stuff could cause more problems than it fixes.

Both rubber spouts are for the same purpose, to drain the fish tank (aquarium). Water collect sin it from the sunroof (the front two drains of the sunroof), from water dripiing down the windshield and from the evaporator. Frequently they are clogged by leaves. If you have a leaking heater core, coolant could come out of there too but the more likely place is inside the cabin and you can smell it.