I found an alternate proposal for the Plombières Agreement:
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Savoy-led Upper Italy would lose Nice, Savoy and Sardinia, but gain Dalmatia and Istria; the proposed kingdom in Central Italy, under a French-backed monarch, would gain Sardinia and Corsica; southern Italy would be under a king backed by the United Kingdom. Personally, I doubt Paris would be willing to part with Corsica and accept a British-backed king in the south, but if France were to choose both the central and southern monarchs (probably Maria Teresa di Borbone for central Italy, and Luciano Murat for southern Italy) this could have a
very small chance of happening. That said, I doubt this setup would last long: Cavour was convinced that, by virtue of the House of Savoy controlling the north, the confederation would've been in his hands; Napoleon III believed that, by having two "French" monarchs in the other kingdoms,
he would've called the shots. By the time Austria and Prussia come to blows, if Cavour's still alive, he might try to do the funniest thing ever, especially if he and Bismarck hit it up.
Unrealistic scenario but, having the two most magnificent bastards of the late 19th century team up would be... interesting, for sure.
Yeah if there was ever a hair-brained scheme, it would be this one. The biggest issue I see is that it likely would not satisfy the interests of the Piedmontese government, the varying ambitions of Italian nationalists, or even the Great powers of France, Britain, and Austria.
Cavour would have loved to oust the Austrians from the Peninsula, especially if it meant empowering Piedmont as well. Plus, he was apparently a big giga-lib in an 19th century way. The problem however is that an Austrian hegemon would be replaced with a French one, which now would have a vested interest in ensuring the new status quo remained as it did. They had their spoils, but anymore would now be dependent on the French government's will. Come WW1 or any major conflict, the Kingdoms would probably have to join a war they would likely not have been in shape to fight. They certainly would not have fixed the Italian economy. All in all, it still would yield many short-term gains and many important long-term gains, but it would be counter-productive in the long-long-term. From what I recall, vaguely, Cavour more often then not tried to both lead Napoleon on but kept him at arms length, which could be easy-peasy-lemon-squeezy or difficult-difficult-lemon-difficult because Napoleon liked to procrastinate.
For Italian nationalists, this problem was basically the same in terms of having a French-dominated regime rather than an Austrian one. But even more problematic, it would have alienated Italian nationalists who wanted to see a unified Italy. Even having 2 kingdoms would have been a major issue. Plus, this meant limited gains in the way of nationalism for the remainder of the Irrendenta, even though there were different versions of it depending on who you asked. The current "natural" borders of Italy, especially in the Trentino-Tyrol would have probably been dead in the water. The fourth shore, greece, and other ambitions would be practically impossible. Keep in mind, many of the Il-Duce's beliefs on an Italian Empire manifested from older Italian ambitions that came to be even before Italy was unified. Colonial Empire might even be impossible, not just because it would require much needed funds from a divided budget. I don't know if French intentions would have prevented or accelerated some of these, but I'm sure there's a case for both.
Austria of course would not like this, but Napoleon and Cavour would've fought them anyway. However, France and Britain would not neccesarily have much to gain from accepting this either. France would basically have to fight for Italy. Even back then, Italian generalship as well as other facets of the military were sub-par at best. If the great war still happened with this agreement, France would likely have to divide their forces between the Germans and the Austrians (not equally but they needed a great deal of resources to deal with Germany), and it's unlikely they would have had major success since the Isonzo river was an excellent defensive position for Austria. It's baffling they believe they would ever march to Russia through the Danube basin anyway, especially if Austria allied with Russia. They could at least provide additional manpower, though they will bankrupt all their client states doing so. Napoleon would also not have guaranteed his "French lake". Italy was an important stepping stone, but he would still be required to match Britain in naval strength, which adding Italy would never have done. Naval power is expensive, and neither French finance alone or added Italian finances would likely have sustained them long-term, or guaranteed other mediterranean nations like Greece, the Ottomans, Morocco, and more importantly Egypt.
As a side note, Britain would NEVER agree to something like this. Getting to choose a monarch is not something they would have accepted without additional compensation, especially since they took threats to naval power of all kinds very seriously when resources were available.
I think this is the original proposal.