Going down the list...
Yes, I had the car plugged in to the 240V charger all weekend... although I've read that this doesn't help the 12V battery.
When the car was dead, yes, everything was off. No lights, no LCD glow, no indicators, no blue power button light, nothing. No response to any buttons, etc.
As for model, I have a 2012 with Nav, Bose, etc. (See sig). Not sure the year/month of production. Voltstats shows me as #2012-09289.
I've had the car about 2.5 months. I've racked on 7400 miles in that time frame due to my lengthy commutes. (I do get to charge at my destination, usually, and sometimes using 240V).
I do have a Bluetooth phone linked, but, I had not made or received any calls on it prior to parking it for the weekend.
Didn't do anything different, really. Just plugged the car in as usual and let it be.
I do have a charging schedule set for Time-of-Use metering to charge in the late evening, but, I doubt that's related.
As for the battery being undersized... *shrugs* Just my opinion. Not sure what could have drained it so quickly. From the time I plugged it in on Friday to the time I got up to leave on Monday was only about 64 hours.
Volt #2012-09289, Black, Premium Trim Package, Bose Sound, Navigation, Jet Black and White Leather