I don't think there is anything sinister here. Just a bumbling amateur hour show. But that's what you can expect when you send essentially untrained people knocking on strangers' doors. Sheesh!
The "brother" was accompanied by a "sister," albeit in the background. They asked for the girl, whose age they might not have known, by name. The man should have immediately identified who he was and that they were JWs. When asked by the parent, he should have immediately revealed how they got the girl's name and why they were calling on her specifically.
I would hope the man was trying to push the hysterical woman's camera out of his face and it just appeared that he "slapped" her. I would be very surprised if he intentionally stuck her.
I agree with others here who say the JWs probably came by the girl's name innocently. Most likely, she either lied out of embarrassment or someone set her up as a prank.
I think this a bunch of nothing, blown way out of proportion, maybe by someone hoping for some kind of settlement (ie., money).
Geez, guys, making a big deal out of this kind of thing tends to make people take us less seriously when there really is something to expose. "See! It's all apostate-driven lies!"