Someone wiser than me will have a different perspective to share, but here is mine.
I don't think you need to verbalize a boundary around the divorce threats.
Personally, I would simply say "Yes, you've said that before." and nothing more.
He is trying to get a reaction from you, stop giving him anything to feed on and let him know you see his constant threats as idle.
I don't think house rules about greetings will work or give you what you want. I might just tell him "Our lack of communication puts such an air of tension in our home. For our daughter's sake could we agree to be polite and greet to one another? If you are not willing to do this, I understand but I am going to continue treating you the way I would like to be treated." Then do it. Say Hello. If he doesn't respond that's on him, not you.
It sounds like you two don't have much of a relationship at the moment. Do you think he would agree to see a therapist together so that you have a neutral third party to help you both communicate your needs? At that point you could better communicate that you are willing to work on your marriage but would like to see him seeking help for recovery.