Assuming the witness statements are reliable then the chances of him not being involve are astronomical. Sounds like the police were very interested in him years ago but they did not have enough evidence to arrest him. According to witnesses Lynne was standing at the bus station in full view of Durst's store and sounds like she even bought pruns from his store and so Durst would mostly likely have interacted with Lynne. So this begs the question "if Durst didn't have anything to do with her disappearance then why didn't he come forward as a first hand witness at the time? That's circumstantial evidence right there. Durst may have even killed his wife, best friend and neighbour later on to cover up because he had let things slipout to them about his past crimes and so they knew too much.
Killing probably got easier for him each and every time. He got away with it, so he knew he could do it again. And again. The only real questions are: How many others were there? And was Lynne one of his victims?
Knowing how far from the actual murder locations his victims were found, might be a clue as to where Lynne's body might have been placed.