I still feel that the most likely explanation is the one Lancs police still believe in - that Nicola fell in to the river.
Far too much is being made by the media - prompted by a certain widely quoted "expert" - of the failure to find a body. But just a quick Google of some fairly recent cases shows that it's not at all unusual for a body to remain undiscovered in a river for two or three months.
Just last week, Russell Tomlinson was found in the River Weaver in Cheshire over two months after he fell in. Police divers had failed to find him. He was found about a mile from where he was last seen.
In 2020, Mark Maclay fell in to the River Wey at Farnham. Police divers and Peter Faulding's team failed to find him. He was eventually found over three months later, just a few hundred metres from where it was believed he had fallen in.
In 2017, Mary Yap fell in to the River Teme near Worcester. Specialist Group International spent several days looking for her, including a sonar search. They found nothing. She was eventually found seven weeks later, about ten miles downstream from where it is believed she fell in.
So clearly it is not at all unusual for a body in a river to evade professional searchers. The fact that no body has been found yet proves nothing. Yet if you read the papers, you'd think it was impossible that she is still in that river.
It's easy to see why the police are sceptical about the other two possibilities (abduction & choosing to go missing). This happened in the middle of a fairly small area (about 0.2 square kilometres) completely bounded by a river and a busy A road. The area has a steady trickle of dog walkers and others, and all exists are either covered by CCTV or (in the case of the river path exit on to the A586) would be nigh on impossible for an abductor to remove a woman from the scene without being seen.
So despite some of the ridiculous criticism of the police, it seems to me that they are simply applying Occam's Razor here - a fall in the river is by far the most plausible explanation.