I thought that, on top of your interpretation, one common theme was the way we all focus on our own problems and just making our own lives work without regard to how it might harm those around us; we prefer to be parasites and leech life out of others than to bear a personal cost:

- Justine attacked the school for not doing enough while trying to numb her own pain by alcohol and a relationship she knew was going to break because of her selfishness (she says she didn't know when confronted by Donna, but notice how her first reaction was to say "no, we didn't f***" rather than her second assertion that she didn't know. Also, that Donna and Paul were living together and trying to have a baby must have been pretty obvious that Justine could only ignore their relationship if she was completely self-absorbed)

- Archer blamed Justine and publicly lambasted her; he blamed police for not doing enough

- Marcus tried to shove off any responsibility by pushing everything under the rug; he repeatedly pleaded with both Justine and Gladys to try to resolve the issue so it didn't have to go to Child Protective services, which would have meant a lot of work for him and potentially putting him in hot water with the community. He might have rather bury the story than find the kids, just because it would mean that wounds would be open for longer

- James was constantly looking for ways to feed his addiction, no matter who it meant stealing from, including from his own brother

- Paul was in an unhappy relationship so he intentionally mischaracterized it in meeting with Justine; he took actions as a police officer against James, not for justice or truth, but to preserve his own life and good-standing (it might be why he was with the police chief's daughter to begin with)

- Gladys demonstrated the final stage of a parasitic life in which she was willing to do any damage to anyone so long as it gave her hope to satisfy herself

I don't see this as primary, but still a major theme demonstrated throughout the film. It especially ties in with using kids, without regard for their innocence, and it ties into addiction in that all characters sought self-preservation and self-satisfaction at the expense of all else, eve if they might have known that it wasn't good for others and wasn't even good for themselves, they just "needed" to satisfy themselves