It's been awhile since I've read this. Rereading about it now, it seems like a conceptual mess. I also have this sense, hard to put into words, that it is written from a perspective of socioeconomic privilege and implicitly assumes a certain freedom, or has as its focus certain concerns that reflect that. It comes across to me now as narrow-minded and lacking in understanding of the broader human condition and divers…
It's about the question of how a man should live, and how should he choose to act - which presumes the availability of reasonable choices, the practical ability to make these choices, and considers the effect these choices have on the community. Yes, slaves or many other less-than-free laborers of classical Greece are not the target audience of this book - as they don't have the option of freely making many of the choices in their life, it's kind of worthless to discuss what should they choose to do if they can't.
It does not argue about how do develop virtues in others, it does presume taking some responsibility for others and being virtuous "on their behalf" (a very paternalistic approach) - it's reasoning about how to best use power and agency assuming you have it, and if you don't, then presuming that obtaining power and agency is the first prerequisite step, you can't make virtuous choices if you're not in position to choose, you can't make virtuous acts if you don't have the ability to act with meaningful influence. Without free will there's neither virtue or vice, just acts that are outside of ethics.
It's not a conceptual mess, it simply argues for a moral position with which (it seems) you strongly disagree, just as it would disagree with yours. Coming back to the issue of socioeconomic privilege, Nichomachean ethics pretty much argues that you have to have socioeconomic privilege in order to be ethical according to these standards. If you're a slave then you are neither ethical or unethical, the whole concept does not apply to you; It makes an argument that the concept of ethics (in the limited, narrow definition of ethics used there, which substantially differs from what we consider ethics in the modern world) starts to make sense once you're above the "food and necessities" level of life and have the liberty to act freely and deliberately make wide-ranging choices with significant influence; then you should study ethics-as-in-the-science-of-making-these-choices.