I wish we had some smaller details.
-The initial taxi driver from Welly; how he explained how Libby was, coherent, mood, actions etc.
-"Distressed" sitting on the bench. In what context? People are used to seeing drunk students and others around there, most vehicles would not stop over a girl just sitting on a bench. Can falling over on the bench be described as distressed or simply drunk? There is no explanation as to exactly what is meant by her actions being "distressed"
There is no evidence of intruders or people spotted around her house to make her leave without entry. Nor do I think her leaving to find some other students or place to go to is plausible. Just because one is a student doesn't mean they know all the others and from accounts, Libby is reserved and this isn't what she would do.
I believe the most simplistic version is either she lost her keys and went to wait in a safer area on the bench or she possibly went to get some food at a takeaway as most drunk people do and it's likely after Welly her and her friends get a takeaway. Then she stumbled to the bench and felt drunk so decided to sit down for 5 minutes first.
I do not believe she would then walk into a dark area or a park, no matter how drunk. Nor would you run in there being chased as though this was some B grade horror movie in which people make really silly decisions. You would scream and keep in the main public lit areas.
The intermittent screams were a vixen fox. No would be killer chases someone and allows them to keep screaming, the risk is too high, they would turn around and leave. Nor would they be assaulting someone for 15 minutes again allowing them to make a noise for others to hear. You don't fall into icy cold water and have the ability to keep screaming. Your breathe is taken away combined with the energy expenditure lost struggling to get out.
The single scream and slam is the possible abduction. Vehicle reported at the park area is the abductors going to a dark secluded place for a sexual assault. Possible strangulation of victim, body dropped into the park and vehicle driven out as not to arise suspicion of a car being there for an extended period. Abductor than re-enters the park to dispose of the body in the river and hide items in the bushes and runs out as witnessed.
This is the reason why other areas and only the park are being searched because the vehicle does not leave the area according to cctv and evidence.
The bushes searches could be due to clothing items discarded that would have floated on the water.
Sorry for the long post!