I think it's possible ex cop did something dodgy, held back that info and it ends up being relevant but he didn't think it was? So he's fucked up, not realised yet the relevance to this case and that gets revealed in last ep?
Yes I can well imagine murdered twin denying knowledge of murderer/event, murderer not knowing she's an identical twin not believing her and that enraging them.
Yes I thought it was spouse of killer in s1 from quite early on too (now I've been reminded). May watch after this series finishes but not now as it'll just confuse me!
Lack of body problematic yes, but given the writer is positing there WAS a big investigation (as I said before, in reality I think this would be unlikely. I think she'd have been written off as a runaway, or troublemaker that got into drugs etc) then it's bloody incompetent that the main suspects in the series weren't even identified as being potential witnesses! Let alone interviewed!
Hogtini to keep it realistic, it's very difficult to time a death even a few weeks after. Essentially the sooner a body is found the easier it is to establish time of death - but the current science is not infallible. Almost 20 years later it'd be virtually impossible. They're basing when they think she died on when she disappeared. At any point, extremely difficult to establish if not a standard funeral, the time lapse between death and burial decades later. Things used to establish time of death - state of decomposition, insect activity, microbial activity...have had too long to change. (I know way more about this stuff than I should).
Frighteningly easy for a body to be buried under a road or building being repaired/built - it's why it's a favoured method used by gangs for "disappearing" people.
There's even recognised procedures for engineers to act and report upon finding remains during a demolition or when they're deepening foundations and find them.
With a hi-viz jacket - passing drivers think killer meant to be there, without - possibly barely notice, think they've lost something?