Ideally, a state should be ordered to God. However, I hardly see nationalism as incoherent. On the contrary, its lucidity is what makes it such a powerful force against the liberal imperium.
As to the question about how disputes between competing nationalisms are resolved, how are disputes resolved between any competing groups? First they talk about it and, if that doesn’t work, they fight. I don’t see this as an argument against nationalism.
Finally, I do think there needs to be a distinction made here: the nationalism of large peoples is different than that of small peoples. That of large peoples has a tendency toward chauvinism and the deification of nationalist ideas…The nationalism of small peoples is necessarily a reaction to their subjugation and consequent assertion of sovereignty over their fate. As such, it is not a political philosophy arbitrarily chosen by an established state but the very reasoning for a state before statehood.
Hakob Manandyan expounded upon this distinction in 1917 in the case of the Armenians.
Indeed, the very fact that it arose in tandem with liberalism is the reason why it is such a powerful force against it: because it derives its legitimacy from the same secular, liberal foundation.
However, a significant difference, as can be seen in its nascent form in some countries, is that nationalism, unlike liberalism, allows for that ideal goal of a state ultimately being ordered to God.