Big time plotholes *SPOILERS
1. Cops know enough to raise an eyebrow about suspects conspicuously trying to establish an alibi. Isabelle goes out of her way on THREE occasions to make herself remembered by people.
2. Isabelle is supposedly on the verge of passing out or ODing from sleeping pills and the cops don't even take her to the hospital for a blood test OR possibly a stomach pump? That's quite irresponsible.
3. Isabelle writes the address on the envelope of the anonymous letter/documents with her own hand. A handwriting expert could most likely draw comparisons and wonder why the OTHER main suspect mailed it and why.