But I DO think that the swinging lifestyle here is responsible for about a 100-timesing of the risk vs their neighbors and peers who do not engage in this sort of thing.
As to the 17-yr-olds, and possible attempt of the VD's to get rid of Westerfield --this needs Occam's Razor. The VD's IMHO were lying about Westerfield almost before their daughter was even missing, and continue(d) to obsess about him and make statements true or false, arguing over a dance or not a dance, a jacket or not a jacket, ...not likely if he were just a pest, a former acquaintance they were trying to shove away, at the time this happened.
Looks more like things had just ARRIVED at cooking temperature, IMO, if such elaborate timeline and do-we-know-him lies (or apparent lies anyway) had to be constructed...