I don't see many sources refer to Tsar Nicholas II and his regime as fascist, but they seem to fit the bill: right-wing authoritarian nationalists. Would it thus be correct to refer to the regime as fascist even with his, albeit ineffective, concessions (Duma, etc)?
In short, no. There were so many differences between the late Romanov state and the fascism of the interwar period, that the two state systems were very different animals. The specifics of Fascist political ideology are notoriously slippery and ill-defined, but there was a set of basic principles within fascist movements that were absent from late Romanov political culture.
For one thing, fascist movements were very much ideologies of the "now" and at times displayed an almost fetishistic worship of modernity. This fetishization of technology often coexisted with fascist celebrations of a mythic past, but this "historical" restoration (and much of this history was invented) was inseparable from modern aspects of the fascist movement. In contrast, the Romanovs at best had fraught relationship with modernity, pushing for the development of infrastructure and modern armed forces, for example, but never claiming to be a "new" political force. Under Nicholas II, the arch-modernizer Sergei Witte often found himself sidelined by a tsar and a political establishment that was inimical to change. By the same token, Nicholas II's own would-be Bismarck, Stolypin, found that the tsarist state found his proposals to modernize agriculture and transform the peasantry into a nationalist bedrock (he termed this a bet "on the strong and sober") often ran counter to the interests of the nobility and the crown. Stolypin was more useful to Nicholas II as an enforcer of tsarist prerogatives post-1905, than as a white revolutionary seeking a conservative transformation of the Russian polity.
The leadership cult of both fascism and the late Russian empire has some congruence, but upon examination, the differences far outweigh the similarities. The fascist leadership cult stressed not only was the leader a man of supreme talent, but also was an individual who rose from humble background. One of the popular election and propaganda slogans of the Third Reich exemplified the social leveling espoused by this form of a leadership cult. The poster showed Hitler in a line of succession of Frederick II, Bismarck, and Hindenburg with the slogan "what the King conquered, the Prince formed, protected by the Field Marshal, saved and unified by the soldier." As the transition from royal titles to the simple "soldier" showed a marked eschewing of birth and title in favor of action. This was a marked contrast to the leadership cult of Nicholas II. Like many of his contemporary monarchs, Nicholas II tried to thread the needle of presenting the Romanov autocrat as a remote figure blessed by God and as a bourgeois pater familias for the body politic. Unlike Victoria or Franz Joseph II, Nicholas II was never really able to thread that needle, and often leaned much more on historical legitimation than his contemporaries. This focus on the sanctity of the imperial office began to pay less dividends as Russia lurched forwards into the twentieth century and the autocracy appeared much more out of step in a rapidly changing world.
The inability of Nicholas II to form lasting political alliances with these new forces underscores another major difference with fascism. One common principle of fascism was an embrace of an extreme nationalism, and nationalism had very little prominence within the imperial state. This is not to say that the Romanov state did not espouse Great Russian chauvinism or discriminate against non-Russian ethnic groups (they certainly did), but rather that the Romanovs had very little love for nationalism as a political movement. The fate and ultimate irrelevance Russian nationalist activists in Ukraine exemplified the fundamental distrust of the Romanovs towards nationalism, even when done in the name of the Romanov power. The imperial state was happy to use pro-Russian national activists when it suited state power, but alternated between this soft-support for Great Russian nationalism in Ukraine and clamping down on national activists when they threatened stability and state power. Even the Russification efforts of the late nineteenth and twentieth century were much more about ordering society and restoring these areas to an allegedly Great Russian cultural zone. Even here, Russification was seldom schematic and predicated on methods of control and quarantine the largely peasant population from alien influences than trying to create a vibrant Great Russian national culture. The Romanovs also employed various non-Russian servitor estate classes, most notably the Baltic Germans, as agents of the monarchy.
All of the above points out that while fascism aspired to be a modern mass political movement, Romanovism (if it even could be termed a state ideology) did not. The imperial state's attempt to engage the masses was often fitful and reluctant. The existence of the Duma was far from a simple concession made in the aftermath of 1905, but rather a sign of how ill-suited the Romanovs were to face modernity.