Just to be clear, Isabelle's presence isn't canon to the Divorce AU if you know what I mean...largely because if he did find out, the obsession would overtake him again and the whole story would take a very very dark turn that poor Cy and Lynn very much did not sign up for. As a general rule, in the vast majority of AUs, if Alec finds out Isabelle's whereabouts after he loses her he'll stop at nothing to get her back.