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.

Alec would love to ignore Lynn's safeword, but values her as a submissive too much to risk losing her like that...which is also why he respects her boundaries around knives and needles despite his love of them. Alec enjoys pretty intense impact play of course, electrical play, torturing Lynn by pushing her towards orgasm even though she's no longer able to...Honestly a lot of the same stuff as Cy, just more intense in some respects and with less of a dynamic of trust. He likes making Lynn feel that her life is in danger. Eventually he'd also love for her to watch videos of him raping other women, but that might take some work.