your HG is blown for sure. Really stop driving it because you are most certainly damaging your already weak bearings.
I too had a 99 Legacy GT than ate HG, ate bearings and then the replacement block burned oil like a 1975 Ford.
The HG starts out with a small hole that gets worse. The first time it fails seems to be right after a period of hard use followed by a slow down. Then after that it continues to loose coolant and overheating occurs frequently after shorter intervals. This is all bad for your oil quality and too much heat in the critical engine parts.

The cold rad is due to air pockets that prevent circulation of coolant. I would wager that the T-stat is fine.