I am suspicious about the person who said they tied the dog to the bench with string. When the dog was found there was no string on it, so the assumption would be it must have broken free of the string, but there was no string on the dog when it was found, nor was there string on the bench. Therefore the string can't have been broken, so was there actually any string used in the first place? That person has been used to make timelines, but if they are wrong about the string, their timings may be equally unreliable.
I could be wrong, but IMO, about the string which many (not saying you) found suspicious. But as people fish possibly very close to that bend in the river, I've wondered if some sort of old piece of fishing rope or similar was found nearby and was used.