I try to be a "top-down" or hierarchical thinker, so I believe it pays to first identify the basic nature (or classification) of an object/referent--here the NAP--prior to figuring out its detailed attributes. That way you don't spin wheels looking for the length of electricity or the color of gravity.
AZR gave a good analysis in his "line of thinking," and I guess Rand pretty much agreed---two rational sources there. Of course someone else would no doubt have a different analysis---there's no limit to convoluted rationalizations all over the map, coming to all sorts of different conclusions.
Yes...we could judge the respective analyses as to their rationality--truth, correspondence--when set against reality. With enough analysis, one would no doubt come "closer to what reality is," and hence "closer to what should be" than the others.
I'd offer that none of that matters, owing to the nature of the NAP itself. In a nutshell, it's a cognitive existent. It's a judgment. It's a choice. Kent McManigal calls it a "promise" and I think that's very close. In any event, it's a decision of one sort or another and as such, it's going to be whatever the chooser makes of it. This is absolutely true, completely irrespective of what ought to be, and even what is.
It's important for each of us individually to conclude "what's closer to the truth" and so "what should be," but in a social context--which is all any of this is about--it is whatever the chooser decides it to be. That's why IMO it's so critical to get the foundation correct...that ultimately all choices are and should be based on the self-interest of the actor and that it's the actor alone who decides that, for good or for bad.
Most people can't even get the reason for the NAP down right. It's not a simple matter, especially when everyone is so focused on a particular act's effect on the other person, rather than the actor himself. This is particularly tough with the NAP since it involves how we interact with others, duh. It's why I don't think anything will ever change unless and until people understand that it's in their own self-interest not to be a looter, moocher or especially thug. All the other reasons eventually get trumped by some situation or rationalization, and thus the commitment to the NAP gets laid by the wayside. This is inevitable IMO because as long as people look outward for their understanding of the good, they'll never be fully consistent with reality and whatever inconsistency there is, will eventually knock out the decency of the "promise" of the NAP.
I don't know if that makes any sense, but I'm pretty sure that's the way it is. It's also why trying to have some "system" in place before folk make a commitment to the NAP, is a total waste of time IMO. Either they choose that they don't wish to live as thugs, thieves and liars...or thugs, thieves and liars are going to be around until they're eradicated. There's simply no other way to get there, and that's a fact.