I think it's fair to say the chances that she went into the water are slim.
Woman who found the dog has stated it was bone dry (not too long after Nicola was last seen) and Nicola looked to be a fit woman so even if she had some reason gone to the edge and slipped which wouldn't have been to save the dog and which you'd expect the police would be experienced to find evidence of, it's unlikely she would have been swept away considering the dept and calmness and it being low tide. Similarly certainly not deep enough to try to commit suicide by jumping in.
I think that's a red herring.
Could be someone planned it to look like that.
If she walked that route routinely and someone knew her movements?
Easy enough to distract a dog with something smelly to eat downwind and someone to have come up behjnd her if she was wearing headphones while on her phone. Don't know how if someone had taken her, they'd have got her out of the area though.
I just hope for her family's sake that the police had good reason for not considering that line of enquiry and haven't wasted valuable time.