Does Russia have the right to intervene in Ukraine to protect Russians?

In light of the Russian intervention in Crimea-Ukraine some commentators have discussed the matter in terms of Responsibility to Protect (R2P).
    My view is that the situation in Crimea only partly corresponds to scenario set up by R2P-concept. First, the R2P-concept rather concern situations when a state or group of states intervene militarily to protect the population of an other state, not its own nationals. Second and more importantly, a military intervention based on R2P is clearly illegal without a UN security council resolution. The UN-member states, including Russia, clearly rejected in paras 138 and 139 of the UN World Summit Outcome document (2005) military intervention under the banner of R2P without a security council resolution.
   Thus, I believe it is more relevant to leave the R2P-paradigm and instead discuss the Crimea situation according to the following question: Does a state (Russia) have the right to intervene in an other state (Ukraine) to protect its nationals (Russians)?
   The starting point is that the use of force is prohibited under the UN Charter and customary international law. The UN Charter only provides two explicit exception to this rule: 1) self-defence against an armed attack or 2) under authorization of the UN Security Council. None of these two exceptions are at hand in the Crimea situation.

Is there a third, implicit, exception to the UN Charter which would allow Russia to intervene militarily in Crimea?

There are three earlier incidents which may assist us in answering this question: 1) Entebbe (1976), Grenada (1983) and Georgia (2008). All three interventions were based on the argument that the intervening countries, Israel, USA and Russia wanted to protect their nationals. Considering that international customary law is a source of international law, state practice and the opinion of the community of states about the legality of these three incidents arguably becomes relevant. However, first we have to consider the wording of Article 2(4) of the UN Charter which states:

All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations.

American scholars such as D'Amato and Reisman have argued that a military intervention is compatible with article 2(4) even in the absence of Security Council authorization as long as the intervention does not threaten the territorial integrity or political independence of a state. This is a very controversial position because it as odds with the prohibition on the use of force, principles such sovereignty and territorial integrity of states. See D'Amato, Anthony, The Invasion of Panama Was a Lawful Response to Tyranny, American Journal of International Law, vol 84, 2, pp. 516-524, p. 520; Reisman, Michael W., Sovereignty and Human Rights in Contemporary International Law, American Journal of International Law, vol 84, 4, 1990, pp. 866-87.

Nevertheless, let us assume that D'Amato and Reisman make a reasonable interpretation of article 2(4). This means that state A have a right to intervene in state B to protect their nationals, but only if state A does not threaten the territorial integrity or political independence of state B. I would argue that this (very controversial) formula would make the Entebbe intervention 1976 legal because Israel's intervention constituted minimal interference in Uganda's territorial integrity/political independence. The Grenada invasion 1983 is more dubious from a legal perspective because USA did not only save its nationals, it also changed the government of Grenada. While the UN Security Council after the request of Uganda failed to condemn Israel's action, the Grenada intervention was criticized by states such as the UK, Canada and the United Nations General Assembly. Russia's intervention in Georgia 2008 went even farther in terms of disruption of Georgia's territorial integrity and political independence and gathered even less international support.
   Based on the argument made above I make the following conclusion. If Russia argues that there is a right for states (Russia) to intervene in other states (Ukraine) in order to to protect its nationals (Russians), this right may only be exercised in a very narrow manner, not threatening the territorial integrity or political independence of other states. Russia has clearly gone beyond the D'Amato/Reisman formula, a formula which already is very controversial.