So, a plausibility check for an idea I've had for a little while (really, a long line in musings of a non-Bolshevik communist victory)
So, basically, the Central Revolutionary Committee of Edouard Vaillant manages to staunch its losses of members to the Boulangistes. (Rochefort, I think, is key. If he can be convinced Boulanger won't bring socialism, a good amount of members will be convinced as well.) For those who don't know, the CRC was an organization much like the Bolsheviks that was supposed to follow Blanquist modes of operation (i.e. coup d'etat rather than popular revolution)
A slightly worse scandal propels Boulanger and the Boulangistes into government, the man on a black horse marching with the Parisian mob to the National Assembly and assuming the powers of government. (A suitably bloody anarchist attack, could work I think, and it isn't like period France is lacking.) 1889 or so.
The Boulangist regime starts out as an empty slate- a sort of further right Bonapartism but slowly begins to swing even further (especially as the CRC didn't bleed members over, and Rochefort isn't there to lionize him to Republicans) I'm thinking political repression and maybe an early version of the "villainous laws" against free speech and assembly.
While he slowly clamps down on internal dissent, the financial scandals of OTL crop up, necessitating an even bigger crack-down, as well as more of the revanchism Boulanger had become known for. This doesn't sit well with Germany and, after only a few years in government, Boulanger finally pushes too far and starts a war. Say, around 1893.
Naturally, France isn't going to do so well. Mutinies as in WWI aren't too unlikely, and unlike IOTL where most of the socialists agreed to the union sacree, Boulanger and his government aren't going to get that luxury. So there will be draft riots, mutinies and a lot of anti-conscription agitation. Meanwhile, many of the more moderate leaders (OTL's Opportunists and even some Radicals) have been pruged from government, leaving groups like the CRC and even the Travail societies of the time to lead the dissent. Expect the many, many anarchists to give their two cents (or two sticks)
France's armies routed and in decay, Boulanger signs an order for a ceasefire and then, in dramatic fashion, commits suicide. His monarchist handlers try to hush it up and seize government, even considering declaring a caretaker government for one of their claimants and trying to fight on. The few Left supporters of Boulanger balk at this and expose the whole thing, withdrawing their support of government even as armies are poised to strike at Paris.
From there, a general decay of order, which results in the CRC seizing control of Paris and various other cities. The lessons of the Paris Commune well at hand, they enforce many of the draconian measures commentators such as Marx and Blanqui believed the Commune should have done. Rallying mutineers and arming a new National Guard (perhaps a Potemkin analogue?) They extend peace feelers to the Germans, who naturally decline.
A few problems here- mainly the question of how it will survive. Perhaps the international socialist movement plans a sympathy strike? (Internationalist tendencies weren't quite as far gone in that period and war weariness will have begun, even on the German side) Perhaps the Germans smart at the sting of smokeless rifles (just as the British IOTL, and with less of a French Army to lean on for support)?
But how does it seem as a broad sketch up until that point? Plausible?