It's a really strange case, in that there doesn't seem to be many things that could have happen, but all of them seem very odd. It's most likely that she fell in the river and drowned sadly. It seems odd that she did, but these things do happen unfortunately.
There was a potential route out of the area, unseen by CCTV but in broad daylight you'd be surprised she just left her phone and dog and went with someone under duress. She could have gone with someone she knew, but again, you'd imagine she'd take her phone and dog.
Maybe she was having some mental health problems and wanted to escape, so left her phone and dog and disappeared of her own accord. I find that very strange though. Wouldn't you leave your dog at home in that case? Wouldn't you take the kids to school and then just go? You wouldn't be thinking logically but I doubt you'd join a work call and then decide to do it.
It's a baffling case. The very experienced diver with the top of the range equipment has said he's never been involved in one like it and if she was in the river he's certain he'd have found her by now.
The phone left on the bench suggests she left suddenly, or if somebody abducted her they made her leave it there as people know you can be tracked by your phone signal. Leaving the phone, unless forced to do so, suggests a sudden panic to make her rush off and leave it as the situation at that moment took precedent over everything else. The dog falling into the water would be one instance but that has been discounted as it was dry. As has the tennis ball theory as I believe she didn't take it. The other possibility is somebody she knew and trusted suddenly shouted to her to come quickly. If said in a state of panic it would make her jump up and forget the phone and everything else to rush off to follow them fearing the worst. I believe there were obstacles and gates in the area that could have prevented the dog following her and returning home. That explains why it remained there in an agitated state. It couldn't follow her and she didn't come back.
Sadly unless she has effected her own disappearance I fear a tragic outcome as I guess everyone else does.