Very well explained and I know due to the nature of your work that you would know from the inside how these things work. I recently watched the BBC documentary missing which showed the effort that goes into finding missing persons considered high risk.
I still think they've been a little negligent just concentrating all their efforts into the river theory. If she was high risk due to mental health problems surely she could have just left everything behind and walked off? She had extensive local knowledge and may have known that the path near her had no CCTV coverage and just took off down there. As a young fit woman in the two hours until the information she was missing was known she could have gone quite a distance. On a few of those programmes even old people had walked miles before they were found. Of course if this was the case then it's possible she would have been spotted on CCTV on her journey, unless she went straight into a vehicle.