I don't think she could have been murdered on that road. It was way too busy, and, even in the dark, it would have been far too risky to murder someone and bundle her into the boot of her car without someone seeing something. I think the suggestion that she was killed somewhere else, and the car moved back to that position seems more plausible.
Another thing that's odd, though...whenever I've watched crime docs in the past, the soil residues on tyres have always been thoroughly examined, as these are often a very accurate indicator of where the car had been. The investigator didn't mention any evidence on the tyres apart from blood. But there would have been traces of soil, and probably vegetation too. Why wasn't the car examined in more detail? Forensic science is pretty sophisticated these days.
I reckon Roger did it. I think he murdered the victim someplace else and drove the car back to the station road, hoping the car wouldn't be noticed (who takes note of cars parked near stations)?.
He got his wife to say he was with her, not realising that her evidence wouldn't count for anything. He wasn't cunning enough to carry on ohoning/texting the victim after her death. He forgot about affairs he had, even though he was already on police record saying that he had been seeing other women. Motive? The victim may have been threatening to expose his affair perhaps? Maybe he lost his temper in an argument?
I think he may have been surprised to find that he had been convicted even though there was a lack of solid evidence to link him to the crime. So he thought he'd try it on; he had nothing to lose.