As promised, here is a summary of what was done to my GT two days ago after the cold-weather incident 9(-27.5 C, -17.5F) previously described in this thread.
Symptoms: Drivetrain error message (with inability to start) low battery warning, 'battery discharged/charge battery' message, check engine light on. After about 25 minutes I was able to start the car but lost it twice. On the third attempt, I kept the accelerator pressed down to keep the revs up until it settled out and then drove straight to my dealer.
The battery was addressed first based on the symptoms above and the belief that all error indications stemmed from it. It was put on fast charge as it only held 8V. After five hours, it still read 8V. That led to the decision to replace the battery (under warranty). I would also explain why there was a problem after I had put the battery on a 6-Amp Fast charge for 14 hours and somehow started the vehicle but ran into difficulty starting and the drive train and discharged battery messages after a 3-4 mile trip where I shut the car off. Even with new battery I now just leave the vehicle running and lock the car doors if I am just 'in-and-out" for a takeout coffee as I was that day.
The Check Engine light was still on after the battery change and that was traced to a defective VANOS solenoid. It too was replaced under warranty.
I almost two days I have had no problem and yesterday after considerable city driving I put the battery charger on after arriving home and saw immediately that the battery was charged and the charger stayed in Maintenance Mode so I left it on the Trickle setting.
One other point - when the battery gets low for any reason the system shuts down selected non-essential systems like the heating system (except Defrost), A/C (though I didn't care about that) and any other electrical load it is programmed to in order to save power for starting.
I hope that my experience has proven to be enlightening to any Fester that has the Regenerative Braking System.