This is just my rambling thoughts after years of reading anything and everything to do with this case. AJMO.
I can't get past Shirley, Jason's mother, and these are the reasons why...
- The whole set-up and the connections in real estate.
- The only person to say Lindsay was scared of her ex was Shirley, why? Seems to me to be setting up a potential suspect to shift suspicion on herself or someone close to her.
- Shirley calls one of Lindsay's friends after the murder putting on a strange accent, similar to "client" who spoke to Lindsay on the phone before the showing. Again WHY would Shirley do that?
- The missing fence piece, the killer(s) must have known this to plan their escape out? How did they know?
My opinion is that the couple who met Lindsay outside is more of a smokescreen and not the actual killers. I believe the killer was already in the house. There was enough time from when the owner and workmen left, to when Lindsay arrived to greet the potential "clients" outside. The killer could have even had time to set up something to avoid a mess or put on something to protect their clothing. They could have brought a gym-style bag (just theorising here) with a clean set of clothes to get changed into afterwards. I think that Lindsay was showing the "clients" around and once entered the area where she was attacked was greeted by the actual killer and was ambushed. The attack itself was so heated as if the person knew Lindsay personally and had a massive issue with her.
Might sound far-fetched but really the whole thing is anyways and to me this is the only thing that makes sense.