Say Murat remains neutral or sides with Seventh Coalition
Would this lead to southern lead Italian unification
How large would Italy be
Would Tuscan still be the base of Italian
https://en.wikipedia.org/wiki/Kingdom_of_Naples_(Napoleonic)
https://en.wikipedia.org/wiki/Joachim_Murat#Napoleonic_wars
This happens to be a sort of pet topic for me and the subject of a (unfortunately interrupted) TL I wrote a couple years ago:
https://www.alternatehistory.com/fo...-kingdom-of-naples-in-the-xix-century.336450/
I think that the survival of Muratian Naples is plausible in two scenarios:
1- as OTL until 1815, then the
Hundred Days never happen.
2- Murat jumps ship as early as 1813 (he had already been in contact with Austrian and British diplomats before the Leipzig campaign) and his contribution in the Italian sector is seen as more decisive.
From what I read, the chief problem wasn't really Austria, but Britain, which wanted Fredinand back on the Neapolitan throne too.
Anyways, assuming Murat stays alive and in power post-Napoleon, things would look decidedly brighter for the Kingdom of Naples than IOTL, for, even though the restored Borbone coopted much of the "French period" reforms, they had ideological barriers towards things like instituting some form of parliamentary oversight and encouraging alphabetisation that Joachim Murat would not have. The regime could of course not afford to be much liberal in post-Congress of Vienna Italy, but I think it would be flexible enough to co-opt the instances that caused the 1820 revolution.
Avoiding that is key, because it means avoiding years of extremely expensive paying of the Austrian occupation force and the exile or sacking of most reformers and competent statesmen in the Kingdom's administration.
When, likely in the thirties, the Holy Alliance system starts showing its cracks, the new Muratian generation might be able to make some gains in Sicily or the Papal States and reform to a degree of "democratisation" similar to what Sardinia had with the
Statuto Albertino in 1848. This would place Naples firmly in the liberal camp and at the forefront of the
Risorgimento.
A more sincere Neapolitan participation to the 1848 revolutions (or something similar which is imho bound to happen) makes it even possible that Austria be defeated in the I War of Independence. But this would necessitate Savoyard participation, so I would not see that Italy would form as a unitarian state lead from Naples, rather as a Confederation with two major powers competing for influence. This could either lead to an improved federal situation when compared to otl or to civil war, foreign intervention and national ruin.
As to your question about "how large" Italy would be, I suppose that in an optimistic scenario, with the South having about 40/50 years of growth and reform more than IOTL the conditions when unification happens would be more equal, thus Italy will have a bigger GDP and more international clout. Of course the North is still more useful for industry and eventually a gap will form, emigration will happen and Italy will still lack iron and coal, but the South will come from a stronger basis and things like
brigantaggio are unlikely to happen.
What could an internally more homogeneously developed Italy do in the second half of the xix century?
It is possible that French aid might be less needed for unification, so at least Nizza might be retained (Savoy looks more difficult and if retained will likely cause more problems than it is worth).
A second (or third) war against Austria is likely, and if it is won (by allying with Prussia if butterflies are minimal in that direction) an Italy which already liberated Veneto in 1848 might "easily" snatch Trento and Trieste, possibly even Istria.
In the colonial theatre maybe Tunisia could be made a protectorate or at least a condominium with France, things in the Horn of Africa should stay more or less the same and maybe something in the Far East might be acquired if we want to be really optimist (influence in Aceh and a colony in North Borneo, maybe some more relevant concessions in China). Nothing more really.
Finally, you asked about the language, but this question had already been solved in the XVI century, Italian will be in any case based on the literary Tuscan from the XIV century, with possibly some divergences in ortography and lexicon, but only minor ones. If the unification is more federal than iotl, as it looks likely to be, this could be a stronger basis for a revival of the various Italian regional languages and dialects, but I don't think that is likely to happen before the XX century. (Maybe the court will keep mostly speaking French and Napoletano, like the Savoia mostly spoke French and Piemontese).