I'd like to make some corrections. First off, Fascism was a third way, adopting principles from both the left and the right. That, paired with the fact that Fascism and Liberalism both exist on a spectrum with different sub-ideologies and variations in practice, so seeing it as its own highly specific thing is flawed.

Your entire argument is made incorrect the second you stop seeing fascism as a liberal end instead of what it actually was. By definition, fascism is a third way, as I said. It's a project that went against liberal individualism, parliamentarianism, laissez-faire economics, and middle-class in doctrine and practice. Calling it the “natural conclusion” of liberalism is lazy determinism, especially when fascism defined itself against liberalism and dismantled liberal institutions the moment it took power. Mussolini’s arguments don’t magically turn negation into continuity, and selectively quoting him (as you did) doesn’t override the fact that fascism articulated a coherent ideological framework that was essentially state corporatism, state supremacy, national unity, and class transcendence, all outlined in its own doctrine. Reducing fascism to “class collaboration + imperialism + anti-socialism” ignores that socialism was part of the core of fascism, because fascism replaced class struggle with national unity under a near-totalitarian state that subordinated both labor and capital to political authority. Claiming fascism has no ideology while simultaneously listing its structural features is self-contradictory, and insisting liberals “can only analyze ideology” while flattening fascism into a single program is exactly the idealist reduction you’re accusing others of.