Plot Point (SPOILER)

So, how did Isabelle know she would have access to Dirk's car, to plant the bloodied scarf there? How did she know he'd be at Christine's at all that night, without an alibi of his own?

Dani tells Isabelle it was the perfect murder, but from a planning point of view, it's not clear what the original plan was, as far as framing someone else...?
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