I've read Websleuths for a while, but only very recently registered. Since I'm local, though, I felt like I could add a few things to discussion that I don't think have been covered, so this is my first post.
One thing I haven't seen mentioned - local politics. In an ideal world, local politics shouldn't influence a missing persons investigation. However, the Commonwealth's Attorney(in many states this official would be called the District Attorney) lost his election back in November. His successor is preparing to take office in January. This means that the prosecutor's office is currently in transition. I don't have any inside knowledge, so I don't know how that transition is affecting things specifically, but I do wonder if the situation could be having an effect on lines of communication between law enforcement and the CA's office. It does seem incredibly weird that there haven't been calls for people to check their property or lots of posters placed all over the area. As far as I know the only "find Katelin" effort out there is a private FB group and that, by definition, isn't going to reach much of the community.
Another thing to keep in mind - Spotsylvania is a county that went from being rural to high-density suburban in very short order. To this day, services are still striving to keep up with that rapid growth. I don't know how many cases the local sheriff is currently working, but that office tends to stay quite busy. If this is a "squeaky wheel gets the grease" situation, it certainly wouldn't hurt for the family to keep checking in with the sheriff's office.
I couldn't find it in any posts here, but has the family contacted Fairfax police? If it's supposedly verified that stepdad dropped KA off at Springfield Mall, the Fairfax PD should be involved. If Spotsy hasn't brought in Fairfax, the family may want to call them.