Whoever wrote the credits for IMDb for this episode needs to go back and watch it again. The credits only list Wilbur Fitzgerald as "the D.A." with no name listed. When Gillespie, Virgil and Althea are in the DA's office, the desk plaque clearly says "Gerard Darnelle."
As for the doubt about this being filmed in Covington/Conyers, I went to high school in Newton County from 86-90 and I can tell you it most definitely is that area. I spent many hours at Porter Memorial Library, AKA the Sparta Police station. The high school on TV was actually a middle school. The house burned in "Ladybug, Ladybug" was a vacant house actually across from a warehouse. I even stumbled across the warehouse they used as their sound stage one night. I watched filming for the beauty pageant episode from my classroom and we were able to watch them between takes from the entrance next to the auditorium.
As for the street signs, I can tell you for a fact that other signs were changed on the square in Covington because they displayed the outline of GA due to being state highways.