Very interesting post, gordo!
Not only do we not know how Jodi came to be behind the wall, to this day, we don't know when she went there. She may have been trying to avoid certain people or, alternatively, she may have been intending to meet someone - that joint that she smoked less than 2 hours before her death has never been explained. She had no smoking equipment on her so, even if she'd had a ready prepared joint with her, she needed something to light it with - or someone to light it for her. She was too early for Luke, who wouldn't have been out before 5.30 - did she intend to meet someone behind the wall for a smoke to kill some time before heading down to Newbattle?
I totally agree that the riderless bike propped precisely at the V point suggests the riders were drawn there by something. Had they been standing next to the bike, then it could be concluded they'd simply stopped for a rest from pushing the bike. If they stopped for a rest, but decided to hop over the wall for a joint, they'd have hopped straight into a murder scene if, indeed, Jodi was killed at that time. Or one of them had an arrangement to be over the wall at that particular place and time?
According to their accounts (and we know we can't place much reliability on them), they pushed the bike up the Newbattle Road, managed to get it going again at the Newbattle entrance to the path, rode it a little way up and then it cut out again. If Jodi was killed at 5.15, the noise of the bike in the few minutes prior to that, while it was still running, may have drowned out any sound from the attack. Did they hear something when the bike cut out and went over the wall to check what it was?
I agree, too, that this wasn't a five minute attack - the distance covered by the blood staining, the "flattened and trampled" area at the opposite side of the woodland strip, the nature of the injuries and the need to strip and mutilate the body all took time to occur.
The clothing aspect is so frustrating - it's impossible to know whether the moving of any the clothes by the scenes of crime officers took place before they were photographed, there's no way of telling whether AW, in going right up to the body, may have accidentally have kicked things into a new position etc.