Dude this is awesome. Would love a TL about this POD.
I don't have the knowledge and time required to do a full timeline, but here's how it
might go:
1814: in order to make the federalist forces in Italy and Germany prevail over the monarchist forces, Prussia and Sardinia have to be weaker. Therefore, in an alternate Congress of Vienna, Napoleon's Confederation of the Rhine is turned into an independent Grand Duchy of Lorraine under the aegis of the German Confederation, and the Republic of Genoa is turned into yet another duchy ruled by a member of the House of Habsburg.
1848: in Germany, the Grand Duke of Lorraine, Karl von Dalberg [1], is crowned Emperor Karl I of Germany in the aftermath of the first Frankfurt Parliament assembly; in Italy, the Provisional Government of Milan is not taken over by conservative monarchists, and Carlo Cattaneo's federalist faction gains the upper hand; the Provisional Government of Italy is formed, including every country that took part in the OTL rebellion plus Mazzini's reborn Republic of Genoa; Germany and Italy are able to contact the rebels in Hungary and work together with them, and Austria collapses; the Treaty of Pressburg is signed, recognizing the independence of Hungary, Italy, Germany and Poland (Congress Poland + Bukovina and Galicia).
1851: after three years of debate, the Provisional Government of Italy becomes the Empire of Italy, an elective monarchy whose constitution shows the clear influence of Carlo Cattaneo (aside from those matters concerning the head of state, it
is suspiciously similar to the Swiss one); as President of Lombardy, the Milanese philosopher is more than willing to let his ATL daughter advise him, leading to societal nationalist policies in defense of the local cultural and linguistic heritage being implemented and emulated elsewhere, especially in Naples and Venice.
[...]
1884: Pietro Paolo Savorgnan di Brazzà does not explore the Congo under the French flag, but under the Italian one; therefore, during the Frankfurt Conference (OTL Berlin Conference) Italy is granted the lands making up the drainage basins of the Congo and Gabon rivers. As in OTL, Brazzà is more of an explorer than a colonialist and, while he isn't able to prevent
all instances of colonial abuse from happening, he sides with the locals more often than not; Italian Central Africa becomes somewhat of a mirror version of the Empire of Italy, with states like the kingdoms of Luba, Lunda, Kongo and Kazembe rising to prominence. Colonial tensions between France and Italy on one hand, and Germany and Great Britain on the other, threaten to erupt into war more than once.
[1] In OTL, Archbishop of Mainz and then Prince and Grand Duke of Frankfurt under the Confederation of the Rhine; a German patriot, he was also an author and a scholar, close to Goethe and Schiller - basically, he seems
made to be the monarch of a German parliamentary democracy).