I can't help but wonder: if the Rhine Provinces break away from Prussia, could third large German state form in the region with Hanover and others?
More likely with Bavaria as those provinces are largely Catholic.
I suspect Franz Josef probably hoped to install the deposed GD of Tuscany, but Louis Napoleon might want a greater Bavaria as the basis of a "Third Germany", and FJ might swallow it as Bavaria was his ally.
Would Saxony regain the territories lost after Congress of Vienna?
Depends on the completeness of the Prussian defeat.
If a sizeable part of its army gets away, so that it still has some bargaining power, Austria might not pursue that demand. Alternatively FJ might try to get all or part of the Rhine Provinces for the King of Saxony (another Catholic) rather than Bavaria, but I don't know if Nap III would go along with that.