This is my favourite 19th Century POD for Italy, though I’ve never seen it competently done.
During the Napoleonic and Revolutionary wars King Ferdinand of the Kingdom of the Two Scillies and his heir Francis are captured and die in some way. The King appointed by Napoleon, Joachim Murat, defects to the allies as OTL and England decides to allow him to keep the throne and hand over the occupied island of Sicily in exchange for certain promises and conditions.
Through pressure of the Carbonari-led rebellion in 1821 he reforms the Kingdom into a Constitutional Parliamentary Democratic Monarchy. The Holy League rejects this and demand changes where upon Murat stands strong and rejects the pressure. In doing so a rift develops between the Kingdom and the Catholic Church. This leads to further reforms to break the dominance of the Church in the Kingdom. It also redeems Murat in the eyes of many Nationalists.
His son takes over an proves to be an intelligent business manager and directs royal resources to modernisation efforts including railroads and city improvements including a Secular Education initiative, strongly popular in the Northern Italian cities which looked on shocked at the Southern Kingdom’s actions, but steps up the anger of the Catholic Church.
In 1832 after the French move into Algeria, the King acts and invades Tripoli in the middle of a Civil War to back one side. The winner however is deposed and Neapolitan annexation follows. The hinterland is harder to control and takes several decades. But land grants for peasants encourage settlements in the newly controlled territories. It is widely recognised across Italy and Europe as the first Italian colony.
This King dies the year before the 1848 revolutions and is succeeded by his brother. The Constitution is still in place but many areas of the Papal States request and gain annexation into the Kingdom, against protestations from the Pope (though there is no military response). These are the only territorial changes of the 1848 revolutions.
Advocates of Italian unification are divided over whether to support Sardinia-Piedmont or the Two Scillies as the chief state to base unification on. This problem solves itself when the heir to Two Scillies, a tall handsome and respectable prince, marries the Second in line to the Sardinia-Piedmont throne, a charming glamorous Princess. Then the heir to Sardinia-Piedmont dies and agreement is reached to unite the two Kingdoms and to impose Scilly’s liberal Constitution North. This freaks out France and Austria.
The resulting war is a revolution in terms to European politics as Prussia and their Northern German allied states join in support of Italy. The ultimate outcome is a Unified Italy which includes all the Territory of Sardinia-Piedmont, including Nice, Monaco and Savoy, the island of Corsica from France, recognition of Italian influence in Tunisia, and from Austria: Lombardy, Venice, Trento, the Italian speaking city of Trieste and Dalmatia. The newly established Germany annexes Alsace-Lorraine from France and leaves the Hapsburgs with the crown of St Steven, minus Pressberg and Ödenburg which Germany takes.
This sets up a divide of Italy-Germany and France-Hungary going forward.
Ferdinand of Naples had two surviving sons, the future Francesco I and the Prince of Salerno. And after them in the line of succession came the issue of the Infante Gabriele and then the Infante Antonio (younger sons of Carlos III). But let's say somehow they are removed from the equation: if Murat keeps his crown/throne, his eldest son won't go to America (except maybe to visit his uncle Joe at Bordentown) and thus won't marry George Washington's great-niece. Also, I dunno if the Holy League's gonna like the idea of somewhere with all the Bonapartes and half-Bonapartes in one place - it's all well and good when they're scheming for a throne without one; once they've got one as a springboard I would imagine it would be a nightmare. I think it would be a near constant petitioning for the liberation of the king of Rome.
As to marriage partners, I would say maybe Roman nobility, Napoleonic Imperial aristocracy (Ney, Fouché, Bacchiocchi, Bonaparte, Bernadotte, etc) and half-Napoleonic (Baden, Leuchtenberg, Hohenzollern-Sigmaringen) or minor Germanic royalties.