The green cap made the pump weep in my 945 too and it's beenfine since I switched to the black one. It's too high pressure for a turbo imho.

Oh, you really don't want to get into a heater core job until all hope is lost.

Your suspected leak was at the lower heater core neck where it goes through the firewall and you said it ran down on the driver's side of the firewall. The normal failure point for the heater core neck should be at the base where it's soldered to the core and that's inside the heater core box. Leaks inside the heater core box will normally (as in almost always) run out of the air distribution box and out the ducting either running down the lower back corner of the tunnel, out the passenger floor vent or down the rear floor vent ducting. In all such cases the flat section of the underpadding and carpet is the main area to get soaked rather then higher up on the firewall.

Any other leak such as you describe should be coming from the engine compartment, presumably running around the heater core nipple opening in the firewall, yet not getting into the heater core box. Normally that would mean coolant was running out between the hose and the nipple and travelling passed the flat foam gasket that seals the nipples to the firewall. If a heater hose has been pried off, such as for a coolant flush or a heater valve change, then the nipple could easily have been crushed which allows this kind of problem (first hand knowledge there). The lower hose is often the hardest one to work on thus the most easily damaged and without it being obvious. The simple act of pushing the hose on as far as possible and re-clamping it a bit forward can stop a leak like that. Perhaps that's what has happened to you. Or perhaps the Barr's has actually done its job. Either way, just keep watch on the situation for now. If the coolant in the expansion tank starts to mysteriously disappear again then check the carpets, blotting them hard with an old towel to squeeze fluid from any soaked underpadding.

As for the drip of coolant at the weep hole in the water pump, just keep an eye on that area for signs of fluid running down and drying on the pump or the engine block. To help prevent premature failure of the pump, make sure the belt(s) are not overtightened -just tight enough not to slip and squeel on a cold damp day. I had a service department spot a drop of coolant at the weep hole and predict the imminent demise of the pump which had only a few K on it. It never developed into a leak and the pump happily lived on with just a green drop of coolant occasionally seen sitting in the weep hole.
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Dave -940's, prev 740/240/140/120 You'd think I'd have learned by now

After all the testing and isolation you've done, my experience with a similar problem says it's simply time for a new expansion tank cap. Standard cap is green. Some people seem to believe the lower pressure (240) black cap doesn't stress the plastic sides and fittings of the rad tank as much, especially in a hot turbo engine compartment.
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Dave -940's, prev 740/240/140/120 You'd think I'd have learned by now