This is discussed by Margaret Moore in The Ethics of Nationalism. Is national sovereignty worth a single human life? Is a nation justified in using disproportionate force to protect its citizens and sovereignty? Are the Geneva and Hague Conventions fit for purpose and ethical? If nationalism is unethical then is internationalism preferable? Is the liquidation of nation states a prerequisite for human development? I refer to ethics in the human context. The possibility of a wider ethical framework is not precluded. In this context, war appears to be a disvalue.
Your question contains several questions. I'm not sure I can answer all of them, but let's give it a go.
Firstly, ethical has a meaning given to it by humans. What is considered ethical varies from time to time and place to place. So when you ask whether nationalism is ethical, you can't be asking in an absolute sense. Either your question can be interpreted as asking whether I think it is ethical, whether others think it is ethical, or what factors might someone reasonably take into account when considering whether it is ethical. I will adopt the third interpretation.
You need to start with a set of yardsticks for gauging what is and isn't ethical. I take it that most people would agree that fairness, proportionality, liberalism (ie not impinging upon a person's freedoms without an overriding cause) and so on characterise what is ethical. Beyond those broad headings, you can quickly get into areas in which there are strongly held views for and against. For example, is it ethical to own holiday homes when some people are homeless, and so on. Given that, and the fact that nationalism has so many facets and implications, there are likely to be some aspects of nationalism that might be considered ethical and some that might not and others that could be argued about endlessly.
Is sovereignty worth a single human life? That depends. If the nature of the threat to your sovereignty included the possibility that many of your citizens will be killed, then yes you might be justified in taking a life to prevent that. However, let's suppose that your territory included an uninhabited rocky island, and some other country claimed it and stationed troops on it- would it be ethical to bomb those troops? Clearly, arguments could be made for and against it.
Is a nation justified in using disproportionate force? I think most people would say not, but there would be no foolproof way of determining the dividing line between proportionate and disproportionate. The extremes would be easy to categorise. To take the case of the foreign troops on the rocky island, most people would presumably consider the nuking of the other country to be an unethical response.
As for the Geneva and Hague conventions, whether they are fit for purpose depends upon what you believe their purpose to be. One of their obvious weaknesses is that they cannot be easily enforced.
Is internationalism preferable to nationalism? That is impossible to say with any certainty. Yes, it would eliminate some of the inherent shortcomings of nationalism, but it might introduce as many problems as it solves.
Is a liquidation of nation states essential to enable further human development? Not necessarily. To give a more considered answer, you would have to specify what you mean by human development exactly, and consider the question of 'further than what and in which direction?'. Humanity has ample scope for development, and development in certain directions is less hindered by nationalism than it is by other factors, such as systematic inequalities, entrenched religious bigotry and so on.
On the face of it, in the context of human ethics, sovereignty is not worth a single human life. But what if the protection of sovereignty involves the protection of an important principle, such as resistance to tyranny or terrorism? The long-term ethical view may be that the sacrifice of life is necessary to prevent enslavement or oppression. Disproportionality may be unavoidable because of practical considerations. So, war may be justified and whether or not the sacrifice of life is justified depends on the context.