Howdy,
The #1 rule is to use one's good common sense. If the situation looks dangerous, a JW will have no qualms about skipping over the house or if two sisters are concerned about going to a particular door they will ask the brothers to do it.
The buddy method is in part a safety measure. During the field ministry, the entire group pretty much sticks together. if a pair is invited inside one of the houses, the brother in charge will wait or the entire group will wait on the corner until they come out. If it takes longer than 15 minutes the brother in charge will usually go over to check that they are ok. If they are discussing he may give them more time or the JWs will make arrangements for a return visit.
My former congregation here in Los Angeles encompassed a lot of rough neighborhoods here in South Central Los Angeles and neither we nor any other congregations ever had trouble while in the ministry from street gangs or criminals, etc.
The only time I have ever been threatened was during one of the few times that I went in service as a teen back in New Mexico, it was quite rural territory, and the guy yelled at me and my brother to get off his porch and threatened to shoot us. I think that rural territory presents bigger chances of danger since people who live out separated like that feel somewhat immune to the law and also tend to have a greater sense of ownership over their land.
Overall, it is extremely rare that Witnesses are in any significant danger in the ministry. However, your observation regarding the sex offender database is a good one and I had included such a question in the guide that I am working on since a parent would want to ask such a question of their ex-spouse if that spouse planned on having the child(ren) accompany him or her in the ministry.
-Eduardo