Sounds like they were a tad irresponsible but nothing more. I dont see the relevance of the time line.
That's an understatement. I think we can safely assume the rumors about the van Dams swinging is true. We can assume that Westerfield caught on and was observing the house for some time. He made an effort to get with Brenda. As for Damon, his child protective sense was severely dulled, probably anticipating the fun of the evening. Probably was only concerned that the kids were put away for the evening. Probably not thinking about the possibility of a home invasion. Not aware that Brenda was out there ticking off an unstable neighbor.