Yes, I believe you are right that a final decision had not yet been made about who Seth would hire to represent him at trial. However, it also seem very clear that Seth had talked to Barth and his office - Barth already had information from Seth about the case, and Barth knew that Seth might retain him. That creates an attorney client relationship between Seth and Barth, even if Seth never speaks to him again. It triggers certain legal and ethical duties Barth owes to Seth- confidentiality, not speaking to other parties or potential parties who are not represented by counsel, not to accept employment that is directly adverse to Seth if it relates to matters that Barth discussed with Seth.
But the big thing is - Barth did not deal fairly with Kat, and IMO he took advantage of the fact that she had no lawyer to advise, in confidence, that her that her legal may be adverse to Seths and that she should not talk to Barth or make disclosures that could be used to incriminate her. It was Barths duty to refuse to talk to her about the murder and her involvement without first having her consult with her own counsel.
