one of the highest causes of accidental death is drowning from shock/immersion on cold water. I think the most logical explanation at this point is that NB sits at the bench after letting the dog off the lead, leaves her phone on the bench and for whatever reason is near the river bank, who knows why, maybe a ball? Maybe just to stand near the edge? She either slips/falls and lands in the river. It would have been freezing at that time of day. The dog may not have seen hence doesn’t jump in. Undercurrents mixed with shock/panic/potential medical episode would result in drowning. The idea someone else is involved is slim, there were plenty of walkers/witnesses around, broad daylight and it’s not exactly easy access in and out.
It's what I think most likely happened (although I think she might have had a heart attack brought on by cold water shock or banged her head on the bottom - As it'd be hard to struggle so much in that depth of water.)
River Wyre at St Michaels: River level and flood alerts
River levels and flood alerts for the River Wyre at St Michaels, Lancashire
riverlevels.uk
A flow rate of 3.66m3/s is circa 219,600 litres per minute. No expert but I think on a river with a diameter of circa 20m and fairly shallow would be more than enough to push a body along.
JMO.