As applicable to, or in the context of COP decisions and annexes thereto, could informal international law play any role in the broad spectrum of possible outcomes?
More specifically, is there any type of conduct
• Such as statements, or
• The adoption of domestic laws/regulations that have more stringent standards/requirements than standards/requirements set forth in international agreements,
that could be construed as intent to be bound by those statements/laws/regulations on an international level.
In addition to the examples above, are there other examples of conduct that might be relevant?
Yes. Informal international law can and will play a role in coordination of nations’ efforts to collectively take measures to mitigate global warming and to assist in adaptation.
Binding public international law can be created by treaty, i.e. by a formal written agreement between two or more nations, or it can be created by customary international law. Unlike municipal legal systems, the international legal system lacks a legitimate authority having the license to use force against disobeyers of its laws. The lack of an enforcer means that “binding” international law should have greater moral authority than would a municipal law in order to have binding effect.
There is a strong presumption that international law exists with respect to the behaviour of a state only when such state has formally entered into agreement by the actions of individuals who are authorized to represent their state. However, customary international law can overcome this presumption and bind all states, without the need for a formal agreement. Customary international law is very slow to develop but, once developed, can be more binding than treaties, inasmuch that countries can repudiate their obligations under treaties while there are certain principles of international law that are generally viewed to be mandatory. For example, it is generally accepted that the principles of the United Nations Charter prohibiting the use of force have the nature of mandatory international law.
It may be that by ‘informal international law’ the questioner intends to refer to customary international law. Sustained common behaviour of states acting under a sense of legal obligation is necessary for a practice to become recognized as customary international law. Any common behaviour of states probably should be maintained for at least  years before such practice could become recognized as binding international law. The present behaviour unfortunately shows no evidence of there being a sense of legal obligation to reduce greenhouse gas emissions. Thus if behaviour were to change today, it would take at least  years from today before customary international law could impose an obligation to limit GHG emissions on all nations.
However, I believe the questioner is referring to rules and regulations “agreed” at the administrative level among states by individuals who do not have, and who do not purport to have the authorization to legally bind the nation they represent. This kind of informal international law whereby individual administrative level representatives agree on rules and regulations without the authorization of their country the rules are not legally binding. Indeed this type of agreement is by design lacking binding commitment.
But agreements that are not legally binding can play an important role in guiding international cooperation with respect to mitigation and adaptation measures. In international environmental regulation, history has shown that nations are willing to make bolder commitments when they have flexibility with respect to those commitments. And the reverse is true with respect to binding commitments – countries’ promises tend to be far less ambitions when the rules are firm. The logic of this behaviour may be that countries are unsure of their power to control firms and deliver the promised results.
 Note that 20 years is an estimate without any real basis. Please refer to a survey of customary international law focusing on the time period before a common practice became recognized as international law.