Q1. In what sense is NAP a 'principle'? What happens if you break it? What happens if you follow it?
NAP is more of a strategy than a principle (see NAP is one of many strategies, Short-range NAP for individuals). However, if you break it, someone somewhere will surely suffer. Maybe even you (see NAP and children, Two bad policemen, A silicone factory in Shenzhen, China).
Q2. In what situations (if any) is it worth to or ethical to breach NAP?
The answer depends on the moral standard. But if your standard is getting your life needs met, which is a sensible standard for an organism, then it may be worth breaking NAP skilfully when you face scarcity. This is perhaps a sad conclusion (see A counter example for NAP).
From best method to worst: avoid, run, finally deescalate or fight (see Defence against violence, Final word on emotional self-defence, Handling verbal abuse, Non-violent non-cooperation for a stateless society, My own Bill of Rights).
Q4. What other ethical principles are natural extensions of NAP?
Verbal NAP (see Does NAP apply to language?, Verbal warning signals, Cab con), skillful deescalation of violence (see Defence against violence) and intimacy (see Beyond NAP: intimacy).
I like this definition: violence is any unwanted interaction.
Q6. How does NAP spread?
Parenting, non-cooperation, propaganda (see NAP and children, Creating new social norms on a Victoria train, Non-violent non-cooperation for a stateless society, Taxation-is-Theft-Notes). And building new and better social networks (see Building voluntary social networks, What I want from the movement, Book review: Radical Honesty, How to change things).
It is many different things (see Which type of violence is the state?).