There are no signs of slipping.
Her dog didn't stay with her (which is what dogs usually do)
She could have fallen in by being pushed by the dog accidentally, so there wouldn’t be any signs of slipping.
They must know she had a tennis ball to say so as not everyone takes a ball out with them.
It could be possible that she had one of those long stock ball throwers and was sat on the bench on her work call and throwing the ball from there.
When it went into the water she could have got up from her seat and bent over to retrieve it and then had a medical episode or the dog pushed her.
The dog running around in the same area would make me think that was her last place as like you say most dogs do do that.
The dog would have sniffed the phone and bench which could have easily knocked it off.
If she had a medical episode or the water was freezing and she went into shock then it would be very easy to drown and be carried off.
If she was abducted or pushed then I would have thought they would have taken her phone or thrown it in the river, especially if it was making a noise because she was on a work call.