@TrickyApe @Nuraghe @Historymaster
So I presume you all would agree that an expansion of Lombardy–Venetia into Modena and possibly the Papal Legations of Bologna, Ferrara, Forli, and Ravenna would have been a better path for Austria to take in addition to re-establishing Habsburg rule in Tuscany? That enlarged area under direct Habsburg rule, bordering Sardinia, Parma, Tuscany, the Papal States, and San Marino, seems controlable to me.
This might however also require some border changes, though. The Garfagnana, Lucca, the Lunigiana, and Massa-Carrara would likely be divvied up in this case, likely just between Parma-Piacenza and Tuscany, especially if Parma were given directly to the House of Bourbon-Parma. Generally I'd say one could just directly attach the lands of the Napoleonic Principality of Lucca to Tuscany, giving them the Garfagnana, Massa-Carrara, and Lucca. Maybe expand it a bit further with (most of) Fivizzano, Viano, and Fosdinovo. The rest could then go to Parma, as compensation for Guastalla going to Lombary-Venetia. Maybe Sardinia-Piedmont gains some small bits of it, too, like Cavanella, Rocchetta, and Suvero. [See this map as reference]
The Pope wouldn't be happy at losing the Legations again, but given the Treaty of Tolentino and how stable that border was from 1797/98 to 1808, it would make sense for Austria to push for that. After all, they were both wealthy and would make Austria's direct Italian territory very defensible thanks to several forts and the Apennine Mountains forming a natural border. Additionally, with their territory expanding southwards, I could see them not using Milan as capital in this scenario, maybe picking Mantua instead? It'd be very centrally located and it's still a very defensible city, with it being part of the Quadrilatero and being near to Guastalla, too, which may just become the fifth fortress in this system.
I also found this passage by Guglielmo Ferrero in his 1941 book "The Reconstruction of Europe. Talleyrand and the Congress of Vienna 1814-1815" interesting and related, since it summarizes the views of the people of Tuscany at the time:
"But Tuscany was a different case. The Grand Duke had ceded it by a formal treaty to France, who, by formal treaty, had ceded it to the Duke of Parma. [...] The former Grand Duke of Tuscany, Ferdinand [...] had in September, just before the opening of Congress, without any authorization, motu proprio, returned to the Palazzo Pitti and resumed the government of Tuscany. The population had welcomed him and obeyed him as their legitimate sovereign. And so he was, if not by letter of the treaty, by virtue of the 'ancient law of posession', which Talleyrand had acknowledged to be one of the foundations of legitimacy, comparing it to the interpretation of common law. An absence of thirteen years had not made the people of Tuscany forget their ancient dynasty and the peaceful prosperity which they had enjoyed under its government. Everyone in Tuscany knew the latter, and no one knew the treaties and the distant powers which, since 1801, had twice changed the regime without in the least troubling to find out what were the desires and interests of the people." (p. 198-199)
Certainly Rome would not be happy to lose Bologna, which was the 2nd most important city of the Papal States and the one with the best industrial base in the state ( but Pio VII could have some grievances on a personal level, given that his native Cesena would also be lost ), but at least the money that in Otl was wasted to control, quell and garrison said legations, here would be totally directed towards the development of Rome, further favoring the restoration of the Papal government, which would be supported by the citizens, very grateful to see important works begin to improve the Eternal City ( considering that in any case a 1/4 of the entire papal tax revenues come from the Urbe, it would be appropriate to spend resources that would have the possible result of increasing this turnover )