Has there ever been mention of a lead?
I can't imagine there not being a lead attached to the harness and if so, where is it?
What did the lady who found Willow running around tie her up with?
Could Willow have been tied to the bench by a lead attached to her harness and she managed to get out of her harness, hence she was now free? Still doesn't explain where the lead is or if it was still attached to the harness? And if it was, then Willow managed to disengage the lead from whatever it was tied to, rather than only wriggle out of her harness.
If Willow had been tied to something (bench) it could explain why she wasn't able to initially follow NB or whoever tied her up, although you'd think as soon as she'd got herself free, she'd leave the bench area and go off in search of someone or somewhere famliar (like the school carpark), not remain in situ - unless NB was very close by, i.e. in the water. And if that were the case, given this dog swims, as many have already said, why wouldn't she have jumped in?
Something's not right about the dog's behaviour. If she knew she could get to NB, then I think she'd try. But if she knew that only a human could deal with the situation, then rushing around looking confused and upset would be more likely. When my dog sees something he thinks is within his capability to deal with (defend/protect/steal a sausage) then he will, but if the cat's vomitting/coughing dramatically/meowing to be let in and I haven't noticed, he paces and pesters me to DEAL WITH IT.