Or if you're out of the car, just press the close door button on the key fob, and squint to see the display with the annoyingly low contrast graphics through the driver's window. The state of charge as a percentage will show for 10-20 seconds on the dash.
? When charging, the contactors are on, so the high voltage pack is available for heating and cooling. Only the ventilation fans (which are one of the largest 12 V loads) and seat heaters (if fitted) run off 12 V. But while charging, the DC-DC is active, so 12 V loads come via the big pack, and ultimately reduce the charge rate a tiny bit. So the 12 V battery should not get depleted while charging.