That is where I am stuck too.
Note - the following is based on the point of entry being located near the bench and where the harness was found.
If we take various experts reports, the body would have sunk pretty much in the area where it entered the water. But, that scenario presumes immediate drowning and the body sinking.
The only way I can make it work is if N was knocked unconscious (there was mention of rocks near the point of entry) and somehow landed on her back. As she was still breathing, she would have floated for a while, and may have made it over the weir. I seem to recall that the tide was going out at that time? I am not sure whether this is a viable scenario.
The alternative is that there is a 'deep hole' near the point of entry and she sunk into it, only to surface and make her way over the weir as the body de-composed.
If she went into the river after the weir, the river is tidal so the body will move more readily with the currents produced by the tides.