I'd have to reread - but wouldn't they have a good chance to get Justine on Demonreach either way? Thomas gets away with the attempt, successful or not, he is looking to hide. She gets a visit. He gets caught/killed in the attempt, successful or not, she is Scared, afraid of retribution, and turns to her (almost) brother in law for protection for her and her baby, his niece/nephew. If Harry hadn't put things together - she's there either way