1. Yes. The Communist Manifesto noted, "Since the proletariat must first of all acquire political supremacy, must rise to be the leading class of the nation, must constitute itself the nation, it is so far, itself national, though not in the bourgeois sense of the word."
Marx wrote in the first draft of The Civil War in France that the Paris Commune "was the first revolution in which the working class was openly acknowledged as the only class capable of social initiative, even by the great bulk of the Paris middle class—shopkeepers, tradesmen, merchants—the wealthy capitalists alone excepted." Marx concludes, "If the Commune was thus the true representative of all the healthy elements of French society, and therefore the truly national Government, it was, at the same time, as a working men's Government, as the bold champion of the emancipation of labour, emphatically international."
2. The Great Soviet Encyclopedia notes that socialist patriotism "harmoniously combines love for the best national traditions of one’s people with a selfless devotion to socialism and communism and with a respect for other peoples. Socialist patriotism is inseparably connected with proletarian internationalism."
Sun Yat-sen, Lázaro Cárdenas, Nehru, Yasser Arafat, and other representatives of what can be called "left-wing nationalism" were not Marxists. They may or may not have recognized a link between patriotism for one's homeland and a "brotherhood of man" or some other wider humanistic aspiration, but otherwise were concerned mainly with the consolidation of nations and/or removing the shackles of foreign domination.
3. I don't know what is meant by "idpol." Nationalism has existed for over 200 years. It'd be pretty awkward to use that very modern term on figures like Thomas Paine, Simón Bolívar, Giuseppe Garibaldi, Abdyl Frashëri, Kim Hyong-jik and Patrice Lumumba (among innumerable others.)
And since "idpol" seems to be used as a term to describe those making arguments against Marxists, then no, socialist patriotism doesn't qualify as such.
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