The reference to the Main Line by the victim's mother is said in terms of where the debutantes came from: "the Main Line [and] Chestnut Hill". It is not suggesting or claiming that Chestnut Hill falls within the Main Line. It is saying that debutantes typically came from BOTH of these "old money" locations.
When Rush and Jeffries are first talking to Travis (about 10:50) in the art gallery, Travis says to the police, "I've already got a Javert watching my every move." This is a reference to the novel 'Les Miserables' by Victor Hugo in which a policeman named Inspector Javert spends his entire life tracking down a convict (Jean Valjean) who ran while on parole.
This is the first episode Tracie Thoms appears in the opening credits.
Emma Vine was murdered on December 14, 1968.
The "Main Line" is another name for the western suburbs of Philadelphia. Chestnut Hill is a sub-region of the Northwest Philadelphia district of Philadelphia. In other words, Chestnut Hill does not fall within the Main Line.