Can't be sure, but presumably the bug in the 451 that caused the 12v battery to go dead, then the HV battery with it, has been fixed in the 453. If this is fixed, then the HV battery will maintain the 12V battery during storage. This maintenance will only discharge the HV battery a very small amount over 4 months.
As far as checking if you can charge through the 12V "cigarette lighter" socket, see if it has voltage (it will power a device plugged into it) when the key switch is off. The answer is likely "no". I have not seen a car where the cigarette lighter socket is "unswitched" for many years. On my 451, I ran charging wires from the battery to under the service flap (hood). This allows voltage checks and charging of the battery even if the doors are locked.