Is there any clear instances where a state i.e government of a demarcated territory with a monopoly of legitimate autority million of ppl. has credible ethical grounds for opting to obey Instead of do

International and Domestic Law
Name
Institutional Affiliation
Various instances make a state to opt to use international law rather than domestic laws. In some countries, there is political instability, and thus the international community has to interfere with internal affairs so as to ensure that it protect the rights and citizens. When the international community is protecting human rights of citizens, it applies the international law. For example, when there is political instability in a given country the United Nations intervenes so as to provide basic necessities such as food and clothing to the affected.
Another instance when international law has to be used instead of local constitutional law is when a national issue is likely to affect other nations in the world. For example, a government may decide to negotiate a border dispute with other countries. In this case, they will have to face an international tribunal that will help in the negotiations process (Kalensky, 1971). Additionally, when the national government is unable to deliver justice in their countries using internal structures, they are free to seek the assistance of international justice body such as international criminal court.
There are various instances when a state may opt to obey national law instead of international law. First, when there is a conflict between the international law and the local law, the latter prevails. The legitimacy of the national constitution belongs to the people and thus in case there is a conflict the will of the people prevails.

Moreover, in instances when the national government is not signatory of international treaties they tend to use their local constitutional laws to solve disputes. For example, USA is not a signatory of the Rome statutes which creates international criminal court. If there is a case against humanity within USA area of jurisdiction, the local, national law will be used to prosecute the culprit.
References
Kalensky, P. (1971). Application of Foreign Law and Domestic Law. Trends of Private International Law, 268-293. doi:10.1007/978-94-011-9590-4_20