Twenty years ago, I was involved in a case where someone seemingly vanished into thin air. It is one of the reasons why I am pretty sure her partner is not involved, because what he is saying in interviews is exactly how the people I knew responded when their loved one just disappeared in a very short space of time in what appeared to be very bizarre circumstances. Their minds constantly glitched on the situation not making any sense, just as his is doing.
In the case I was involved in, the person was found. But what had happened was both bonkers and yet made total sense in the end. She had fallen from a moderate height, trying to cross from one balcony to another, hit the ground, broken a leg (but not bled) and then for some odd reason, crawled under a parked truck just two feet from where she landed and passed out. The reason why no one could find her was because no-one thought to look under the truck and, of course, she couldn't answer when they were calling out her name because she was unconscious.
Of course, at the time, there were wild fears that someone had broken into the apartment, that she'd been abducted, that someone had been hiding in a closet ... had it gone on for a few days or so, and the lass died under the truck without being found and no one had moved it, no doubt, people would have started saying her parents were involved.
These things always make sense somehow, even if foul play is involved. I have a gut feeling NB didn't enter the water by the bench. The last sighting is in the field; that is where you have to start. There's no clear evidence she was ever near the bench, even if her dog and her phone were found around there.
Likewise, there's no evidence she wasn't near the bench. Maybe she walked past it. Maybe the dog started fussing as the teams meeting ended, she pulled out the harness, while putting her phone in her pocket, and when she got further up the path, she realised she was still logged into her teams meeting, but when she tried to find her phone, she realised she'd dropped it when she was dealing with the dog, and she retraced her steps past the bench (not seeing her phone) and thought she'd just look beyond the gate, but didn't want to take the dog through it again.
Or maybe she did take the dog through it again, and the dog simply went back through the gate at a later point with its harness in its mouth (the gate appears to be dog accessible one way but not the other).
The starting point is the field. From that point, she could be anywhere.