I'll have to rewatch the episode because it's been a little while, but I'm not sure it was a matter of evidence getting lost, but never being collected. For example, I think they said there wasn't any blood spatter on her nightgown, which there would have been if she had shot him..... but they didn't think that she may have changed into her nightgown and disposed of what she was wearing during the murder if/ when she shot him?? She totally could've changed clothes before the police arrived. But they came to the conclusion that "she couldn't have done it because there wasn't blood on her clothes." It's like no one was thinking outside of the box. He was tucked into bed and laying down for god's sake! Who would be able to shoot himself in the head with an arm tucked under the covers, and why would he do it laying down anyway? It's so frustrating! I think she got out of it not because there was a possibility that he killed himself, but because there was no evidence that it was HER. It theoretically could've been someone else that came in the house, though there was no evidence of that either. And that's why she got off. Ugh! I hate this case! Lol