Basle is one hour ahead, which doesn't make all that much difference, but perhaps Janet was an early sleeper and he knew that and didn't want to disturb her, and perhaps he assumed Roxane was out and about.
I am interested in the eyewitness report that spotted Janet driving away from her home at 18:30 that evening. It hasn't been established where she was going.
I'm also interested that the police were convinced or fairly convinced she hadn't been having an affair. I assume because they couldn't find any evidence she had been seeing anyone, but perhaps she had been and they just could not find that out.
The murder really does not seem like a burglary gone wrong. A professional burglar rang the hotline and said that no burglar would enter a property like that. The cars in the drive would also put a burglar off, and it's most likely whoever did this brought the handcuffs with him, which are not items you would expect a burglar to carry. If they were Janet's, it's unlikely she had them on her bedside table for Roxane to see! Perhaps the perpetrator knew Janet but she did not know him -- if he was someone who had been stalking her from afar. But if she was targeted, he had knowledge that she was in the house on her own -- husband away and daughter out. It would have been harder to control two people, and thus riskier.
Why did PB think that the killer wanted to target Roxane as well? Makes less sense to me.