One thing is certain we know specifically which window Kate claims was open, but with Mrs Fenn she hears a child cry but we can't be certain it was the same child for the hour and quarter.
Nor which apartment (s) the sound (s) actually came from. In her statement, she stated that she oftens stays up late. Where was she at the time - patio-side (more logical for a living room) or in bed (possibly road-side or off to the side)? Some people also have a TV and a phone extension in the bedroom, but she doesn't appear to have been asked.
She also stated that she didn't hear anything on the evening of 3 May until approximately 22:30 when she heard a woman shouting (who must have been Kate). People were already swarming around the flat by that time, and I doubt that they were all whispering.
Many elderly people have hearing aids, which I'm told don't always reflect sound accurately. All the more reason to have conducted a reconstruction to verify where the sound could have been coming from and for contacting Edna to check on the time.
Who was above Mrs Fenn? Could the sliding door noise have come from above and not below?