Definitely a valid scenario but a few questions.
Witnesses heard screams in the park and others heard her shouting even before she met PR, so she was inclined to shout that night. Why did she not scream before entering ORPF?
Some people question her ability to make it to the river from outside the park because of her drunken state, she was described as being very very drunk and in a state. Also if she did choose to run into the park herself how did she avoid running into the open section and being spotted on the factory cctv overlooking part of the river bank, considering she ran in the other direction (near the playground) how did she not fall in the large pond instead?
Also if this was the case and she fell into the river accidentally, and PR knew this had happened, why wouldn’t he just admit she fell in the river and possibility admit the rape as this would have meant a lesser jail sentence than being trialled for both rape and murder.
Good questions! I would suggest it could be possible that Libby did not scream before entering the park either because she was semi-conscious at that point or possibly, if she was aware as soon as they stopped that something was wrong, PR put his hand over her mouth during that time and during the assault.
The question of why she did not show up on CCTV in the park would apply to PR equally - we do not see someone carrying someone towards the river or chasing someone. At some point we know Libby entered the water but the CCTV did not pick this up.
I think if PR was anywhere near Libby when she went into the water he would know that he would be accused of having caused that so he would state that he was not anywhere near her.
This is what I think:
LIKELY: PR did take Libby into the park, assaulted her there and then or later put her body in the river. He says he was not in the park to distance himself from the location of the assault.
LESS LIKELY: PR assaulted Libby at the shed and then chased her across the park where she fell into the river.
LESS LIKLEY: PR assaulted Libby at the shed, Libby broke away and ran into the park, PR maybe following a few steps but then leaves and Libby, distraught somehow falls into the river after he has gone.
VERY UNLIKELY: Libby threw herself into the river deliberately.
Would this give us murder for scenario 1, manslaughter for 2, debatable manslaughter/not guilty for 3, not guilty for 4? This may be why the defence seemed to concentrate on 4. Because as I see it there are credible options other than murder and even manslaughter, in the absence of other evidence possibly seen by the jury I can't say guilty.