Going back to basics a little bit i suppose ...
If Nicola fell in the river , having full trust in the initial police response and the SAR teams on site within hours ( as far as i recall ) , further corroborated later on by PF and his team , we can confidently say that, having not found her up to a mile upstream from the bench area and ,at the very least, down to the weir , for the fall in the river scenario to have realistic sense it kinda follows that:
A) She fell in the water and was transported by current past the weir very quickly
B) She fell in the water much closer or even past the weir , for the initial searches to have missed her ( and for this we need context as to why she would even be ther in the first place , presumably with Willow left in the field)
C) The SAR teams and PF's team have simply been unable to locate her in that stretch of river (very unlikely given the scale , length of the search and equipment used )
If none of this apply , fellow sleuthers , we are left with asking ourselves if it's easier to stage a disappearance/fake death or for an abductor/aggressor to carry out their deed and remove Nicola from the scene/area . Willingly , unwillingly or forced to.
And work your way from there .
All of this in my view , in my opinion .