

First of all, I am honestly not answering the question of whether I consider nationalism inherently good or bad. Those are value judgments, and therefore I am not even trying to combine left-wing movements with nationalism. I am only asking myself whether nationalism is, by its nature, a left-wing ideology.
Secondly, regarding the question of whether there are examples of movements that broke away from an existing socialist project under a national banner without becoming fascist or capitalist — there are. The clearest example is Tito’s Yugoslavia in 1948, which broke with the Cominform and Stalin while insisting on its own “national” path to socialism, independent from external dictates coming from Moscow.
There are also examples of this kind of separation within a state itself. The most illustrative case is the Croatian Spring (MASPOK) in Yugoslavia in 1971. It was a movement within the Croatian Communist Party: the reformist wing of local communists demanded greater economic and political autonomy for Croatia within the federation, criticized the redistribution of resources in favor of Belgrade, and advocated a “Croatian path” to socialism. Tito suppressed the movement, its leaders were removed from their positions, and some were imprisoned, but the movement itself was formed as a socialist, nationally oriented movement rather than an attempt to restore capitalism.
And ideologically, one can also split from an existing socialist project, as Trotskyists did, although this does not need much explanation since you already know the example.
I think Lenin’s framework is interesting, but I would be careful about taking it at face value, because the historical practice of Leninism complicates the picture quite a lot.
The distinction between oppressed and oppressor nations is a useful analytical tool, and I agree that nationalism does not have an identical meaning in every historical context. A national movement of a people trying to escape imperial domination is not the same thing as the nationalism of an imperial power trying to maintain control over other peoples.
However, the problem is that Lenin himself did not always treat the right of self-determination as an unconditional democratic principle. He was quite explicit that recognizing the right to secession was also a political strategy something that could reduce resistance to the new state and make voluntary unity more likely. In other words, the promise of self-determination was not only about respecting national rights, but also about creating conditions where those nations would choose to remain within a socialist framework.
This becomes especially important when looking at what happened after 1917. The Bolsheviks proclaimed the right of nations to self-determination, including the possibility of secession, but when Ukraine and other nations of the former Russian Empire actually attempted to use that right, the situation changed very quickly. Ukrainian independence movements, as well as movements among other non-Russian peoples, were opposed militarily by the Bolshevik government. This suggests that the principle was applied differently depending on whether national self-determination threatened the new central authority.
That is why I think the Norway–Sweden example is more complicated than it first appears. It works well as a theoretical example of how socialists from a dominant nation should behave toward a weaker nation. But the situation of Ukraine in 1917–1921 was different: Ukraine was not simply another independent country seeking recognition; it was part of the territory over which the Bolsheviks were trying to establish a new political order. The question was no longer only about supporting self-determination, but about whether that self-determination could actually limit the power of the revolutionary state.
This does not mean that Lenin’s distinction between oppressor and oppressed nations is useless. I think there is still an important point there: nationalism from a position of domination and nationalism from a position of resistance are not the same thing. But the historical lesson is also that even a theory built around liberation can become a political instrument when the people applying it have their own state interests.
So I would separate two things: the idea that leftists should oppose imperial domination and support the right of nations to decide their own future which is a principle that can be defended independently and Lenin’s own political strategy, where the recognition of that right was often conditional on whether it helped or hindered the revolutionary project.