as I watched this:
The agents are concerned about keeping the peace. They are not investigating a crime. Is this part of their job description?
Regarding #4, is the mom accusing the woman FBI agent of lying? This speaks to the point above, regarding the FBI's job description. The FBI agent is saying that part of their job is to prevent violence from occurring. The mom should have asked to see an official copy of the law describing their office to verify the agent's claim. Or, if she had in fact already seen an official copy of the law and the agent had just misspoken then she should have corrected her.
At one point the mom says she wants to continue the conversation because she is "curious." I can see engaging in such a conversation if one is concerned about the peace being broken, but since she has no such concern, there's no purpose in continuing. And if she had been concerned about the peace, she would have already gone to the authorities. So there's no reason for this conversation to have occurred, outside of her wanting to satisfy her curiosity.
She's uncomfortable because he is asking her to give evidence. In fact she doesn't have any evidence to give – otherwise she would have already called the authorities. I think both of them are uncomfortable because there is no purpose to their conversation.
Really, they shouldn't be at her door in the first place. That she agreed to talk to them doesn't make sense, except that she could have simply acknowledged the concern (prompted by the “call in”) and told them she hadn't noticed anything that would cause her to be concerned about violence.