Just an FYI in case your wife doesn't know about the
REST (RESidual heaT) feature...if the ambient temp is below 59F/15C...and the engine has reached normal operating temps...you can use this feature to keep the cabin warm (for a while) without running the engine...in fact, you can turn the engine completely off and leave the vehicle and the blower will continue.
You can run this feature with the ignition in position 1 or OFF. Which means there are fewer electronics running which also means less impact to battery drainage. The seat heaters require the ignition to be in position 2...that means ALL electronic modules are running along.
You obviously can't do this indefinitely...but while sitting/watching the fireworks...you could use the REST feature with engine off (conserving fuel & less electronics are on except the blower & IHKA panel)... then restart the engine when the air begins to cool & use the seat heaters to stay warm.
Again, the more electronics running, the quicker the battery drains.