The Ottoman Empire would be contiguous with Istanbul, like the Roman empire would be contiguous with Rome and the Holy Roman Empire would be contiguous with the Vatican. In fact, the interesting question would be whether the Holy See is in fact contiguous with the Holy Roman Empire. I think it might be.
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The Ottomans existed well before they conquered Constantinople. The Vatican (or Papal States at the time) was explicitly not part of the HRE. They were in many ways opposites; the seats of spiritual power vs temporal power. This tells me you have very little sense of general history.
You also keep mentioning "the framework of Westphalian nation states", which is also a tell since you're confusing two different concepts; the Westphalian system and the concept of the nation-state. These are associated with one another, but distinct concepts.
Importantly, China is not a nation-state. China is a civilisation-state, which is a grander concept as the nation-state is far too European an idea to make sense for China. Both the ROC and the PRC claim to be nation-states, but these claims are somewhat doubtful definition-wise. Regardless, this places the earliest possible concept of a Chinese nation-state in 1912, when the Qing Empire fell (an empire, mind, so by definition not a nation-state).
This also means that Taiwan, which was conquered from the Qing in the 1890s, was not a part of a Chinese nation-state until 1945 when it was ceded to the ROC.
The problem with characterizing China as a nation-state is that it doesn't consist of one nation and one state, it has far too many peoples, cultures and languages inside it for it to be considered that. You'd be doing its diversity a disservice, really. Hence it is a civilisation-state.
Characterizing it as a nation-state reeks of nationalism and imperialism, which is typical for nation-states. Claimed lands are assimilated, either through coercion or force (or ethnic cleansing). A heroic epic is created to turn the birth of the nation-state into something mythical. Wars are fought to establish borders, usually along natural, defensible lines. Interestingly it's a perspective the CPC is keen to avoid (since it's not very "socialist" after all).
Nation-states are nationalist/imperialist in nature because they often violate the concept of self-determination. It is by definition the amalgamation of various similar cultures and peoples by enforcing a shared identity (and making those who don't conform to it do so anyway). The Taiwanese population does not want to be ruled by the PRC for example, yet the PRC claims legitimate governance over the island anyway based on these nationalist claims. Similarly, the Spanish suppress the Catalan identity, the French assimilated the Bretons and the Alsatians, etc... It is this enforced unification of people that is not a very socialist viewpoint, people should want to unify on their own accord.