Brilliant points SS.
1. I agree, if the lead and harness were there surely it would have made more sense to use these. Did they not see them? String seems odd and string can be very think would a dog not be able to break free from string or chew through it? I wonder if it was rope and not string.
2. Was the person who first found Willow walking their own dog? Could they have had some form of rope on them (perhaps and extension for a lead for their own dog)? Was it when the first person found Willow that Willow was distressed or was it when the second person went to the bench (a considerable period of time after) that the dog was distressed? Hardly surprising after being tied up. Did any of them contact the dog warden or the Police given that the phone was also there and still connected to a conference call which had ended?
3. I agree, the period of time that passed and well over an hour is significant. I wonder if the first person who contacted the other one possibly left a message on voicemail which might not have been retrieved right away. Could this explain the amount of time that passed by?