Re: Crazy Ex
1. This is a pretty good idea. Moving on and deciding not to deal with all that craziness is a good idea. If she asks why you aren't talking to her, tell her about how her crazy ex is stalking you and that you don't want to deal with either of you now. It might come back around to where she tries everything to try and get you back.
2. You could do this, but I wouldn't recommend it. Never help out a guy who's trying to get at your target. That's like trying to help an insane man who is trying to kill you by walking right up to his doorstep.
3. This would be at least a bit better than #2. You can tell her "Hey, your creepy ex has been stalking me". Make it clear that you are annoyed by him and that it is currently making her more unattractive because you have to deal with him.
4. This is similar to number three, only with the prospect that you are befriending the enemy. This would not boil over well. If he figures out that you talked to her, then he will just simply become angry at you. If the opportunity comes along, he will try to cockblock you.
There are two possible choices here, depending on how close you are to here. If she is initiating conversations and she's trying to talk to you, then #1 is the way to go. If it is the other way around, then #3 would be the better option. Either way, don't help the ex.
"There is no better than adversity. Every defeat, every heartbreak, every loss, contains its own seed, its own lesson on how to improve your performance the next time." - Malcolm X