IIRC, Napoleon III never looked for major annexations in Italy (except for Savoy and Nice): his plans were centered on the expulsion of Austria from Italy, and setting up three states (plus the papal states in Latium): a kingdom of Northern Italy, under the house of Savoy; a kingdom in central Italy, under prince Murat; and finally the kingdom of Two Sicilies in the south, under the Bourbons. While this final configuration might have been too ambitious, the secret agreement of Plombieres in 1858 was that France would intervene in Italy to make war against Austria, and liberate Lombardy and Venetia (which were to go to the house of Savoy after a plebiscite); also Bologna should be warded to Piedmont. As a compensation, France would receive Nice and Savoy.
This smacks a bit of 18th century diplomacy, and completely ignores the lessons of 1848: as a matter of fact, as soon as the Franco-Piedmontese troops won decisive battles at Solferino and Magenta, there were popular insurrections in the duchies and in the papal states, which quickly forced the previous rulers to leave. Nappy's appetite for war was decreasing very quickly: French losses at Solferino and Magenta had been very heavy, the conquest of Venetia would have required another bloody campaign and the unsettled situation in central Italy was quite worrysome for France.
France signed a unilateral armistice at Villafranca, without consulting its ally.
On the basis of this armistice and the subsequent Austro-French negotiations, Austria would cede Lombardy, but all the insurrections would be put down and the previous rulers reinstated. The cat was out of the bag, though: the former duchies and legations formed the League of United Provinces, with covert British and Piedmontese support, and Cavour was able to play Nappy beautifully (a republican state in central Italy could have been a problem) and ultimately the kingdom of Italy was proclaimed.
This brief recapitulation of OTL events has been set up here to show that unification was very hard to derail.
The unification has not to come necessarily in 1859: Nappy might have refused to sign the agreements of Plombieres (even if from his point of view it looked like a very good opportunity to extend a kind of French protectorate to Italy). Notionally, it is also possible that Austria will be able to keep the staus quo in both Italy and Germany (both of them: a German unification promoted by Prussia would necessarily go through a war with Austria and this would be the spark for the Italian war), and to keep the Hungarians in the fold and to suppress Slav aspirations: this was Metternich strategy, and failed in 1848. The Austrian empire was able (barely) to survive and fight back. Can they hold everything frozen for another 30 years? I doubt it very much: this is the time of nationalism and popular aspirations, and the more one keeps a lid on the stronger the explosion which will eventually come.