The idea the couple would be home is justified as they had a nice house WITH A GARAGE. There are many things that can happen in a garage and driving down a street of a suburb completely undetected except for a 'hello wave' by a neighbor watering their lawn or racking leaves.
The privacy standards of the USA are a friendly basis for secrecy. I think the couple was using the garage covertly and may even have practiced their route to and from the holiday party.
But, I can easily see them leaving with the weaponry and body armor in the SUV. He went into the party and was dressed appropriately without weapons. He later returned with spouse with weapons and a plan. They may have worn ski masks to hide their identity.
They could disrobe in the SUV and assume everyone seeing them from the waist up would never detect them as the killers. They had a strong reputation of being wonderful people. No one would have a reason to mistrust them.
They could drive the SUV with their cache of weapons and body armor into the garage and close the door. Mission accomplished.
Why would the couple think such an attack would work to change policies that scarred Daddy's reputation? The internet.
I am not stupid, they were obviously destroying their cyber records in anticipation of something, either a search of the premise or they would be dead and baby would not have to know what the parents did. But, I have to ask did they actually believe they were going to be dead. Was the destruction of the cyber records to protect others in that they were dead or was it to destroy any possibility of linking the attack to them if a search of the home was conducted.
If a search was necessary there would be much more to implicate them in the attack than cyber records, they would have gotten caught. But, no one ever accuses criminals of being geniuses.
Have a better day, I really need to finish by Christmas decorations.