Quote: "I'd like to think he won't cheat."
If anybody in this scenario gets committed, it won't be the ex (or maybe not) girlfriend. This poor poster is letting her mind run away with her. And really, play it out. So suppose you, as you want to, managed to get the IP address of the person who subscribed you to Victoria's Secret and it turned out to be her. So what? You certainly couldn't go to court and complain. "Judge, I want this person punished, she's sending magazines to my husband in a clever crafty attempt to break us up by making me jealous!" Can you see how doggone silly this all is?
If your husband wants to cheat, he'll cheat. Bet on it. He may, at this minute, be cheating on you with someone you haven't even been thinking about while you wasted all this time effort and mental energy on the ex girlfriend and what she's supposedly doing to you. See, there's no nailing it all down and making it go away. If he wanted to apologize, he'd apologize. What he's got going is you being crazy in one way and his ex girlfriend being crazy in another way (can you see any common thread here? Like HIM?)
Please forget the legalities of chasing this other woman, who is probably just one among many of his conquests and trophies, and work on yourself and getting yourself to a healthy place. He is right about your not being able to get the IP address and what I'll tell you is that it is NOT important anyway. The less he encourages you to do this chasing of petty details about the old girlfriend, the MORE confidence you can have in him as it is to his credit that he's not encouraging you to act crazy as you seem to want to!