In the roadster, charging and driving is from the same PEM, right? Charging, the PEM runs AC -> DC. Driving, the PEM runs DC -> AC. But, only one way at one time. Hence those discussions about towing a generator to charge while you drive won't work, and the roadster won't turn on while the charge port is open.
Now, again in the roadster, 12V accessory power comes from a 12V converter in the ESS, and that is live while charging (otherwise VMS wold have issues). HVAC operates from the 400V output of the ESS, and is converted (to what?) by the orange box in the front trunk. The roadster can run the HVAC during charging, but can't direct the cool to the cabin, but I suspect only because the A/C can't be turned on without the car on. No electrical reason why not (unlike driving and charging).
So, what's the arrangement with the Model S? Sounds like the PEM and charger are two separate devices.