The witness living by the park who actually woke up and heard the cries said they were loud, franctic and if I am correct they moved location until it all went quiet. Then few minutes later a man (presumably PR) emerged running from the woods out of park...
So that tells us that out of the 7min30sec that PR spent in the park before PR returning to his vehicle, Libby must have been alive up till the last few minutes before he returns to car...also that she was not stationary in one spot but rather moved/run/being carried whilst screaming
Few scenarios are possible...
He carried her towards river whilst she screamed (he most probably scouted suitable locations in advance so he would know where he wannted them to go) then raped her and left her for dead or asfyxiated her in the proces then either threw her in the river or returned on his later visit to do that (the body would be placed very close to river so he would manage in 4minutes)
Or he raped her in the park and she run away from him whilst screaming..then he left her behind and run out of park and returned to his car...Libby would then fell into river herself (not totally excluded) or she dies from hypotermia or trips and falls unconscious and then dies of hypotermia...again patologist did not exclude that completely but it is more unlikely given post mortem did not show signs of dying from hypothermia. Also if he just raped her and then left her in the park why would he wait to run away only after the screams went quiet??? Why not just run away as soon as he can...to me he knows why she stopped screaming. Only after that he emerges from the park (probably knowing that she is not going to alert anyone anymore).
So what makes her stop screaming in that park few minutes before PR emerges from the woods? Whatever it was he was still there when it happened...
Im sorry im thinking out loud as I type