There is no evidence of anybody going through that window. It's no wonder the case is still open. They know it's time to separate the the wheat from the chaff!
I agree that there's no tangible evidence that anyone went through it. Or at least none that appears to have been recovered.
Personally, I can't exclude it as impossible, but I'd place it in the unlikely category.
I'd have expected prints on the outside of the window, but I'm not sure the outside of the window was dusted. But if the person had worn gloves, there wouldn't have been any anyway.
There were unidentified prints on the outside of the blind, but that could have been Gerry trying to see if it could be lifted.
I'd have thought that fibres might have been found - but they could have blown away. So that's a "dunno".
I would imagine that there could have been footprints on the bed, but it was messy and I'm not sure if any would have been noticeable. Or on the chair, but it was wicker. Or the person could have been barefoot.
A large man would no doubt have found it difficult, but a child accomplice or small adult wouldn't have done.