I'm currently reading a book about Louis XVIII which explains a lot about the consequences of Napoleon's fall for France and the new government. The first time the allies were quite generous and forgiving but the second time was a massive burden and punishment for the new government and the people.
Prussia seem to have lead the anti-France and anti-French negotiations, England watched a lot, Austria was double playing between ensuring that France never starts again and not letting Prussia be " the sole protector of the German people " but Russia was the most generous nation of the four. Though it imposed sanctions like the others ( reparations, occupation,...Etc ) it helped France not to be crushed by them. The Marquis of Bonnay declared that Russia was " France's ally " in these post-war negotiations bu why ?
What political elements lead Russia to be such " a friend " ? Surely the friendship between the Duke of Richelieu and Alexander I was not the sole reason ( though it must have helped! ) and the fact that French ambassador in Russia Pozzo di Borgo was getting along quite well with the Tsar was not a reason. Was is it something to do with Prussia ? Russia had to bring Britain on its side to prevent Prussia from getting too greedy with France so it must have been something to do with Prussia gaining more power ?
I'm not sure which book you've read or what it says exactly but you must have misunderstood its intent, Alexander was most definitely not a friend of the Bourbons during the Congress of Vienna. The relationships you note between Prussia and Austria also seems bizarre.
Alexander I had a very low opinion of the Bourbons, viewing them as a weak dynasty filled with ingrates that failed to keep their own throne three times. Since 1812 he'd favoured the more dashing and modern Bernadotte, King of Sweden and Napoleon's former Marshal for the French throne. The idea was of course another absurd invention of Alexander and was quickly shut down by the other Allies, but he continued to try and push Bernadotte over the Bourbons. It was Britain that favoured a Bourbon restoration the most, though Castlereagh was always adamant that Britain not involve itself in the internal governance of France - a notion that ended when Wellington all but put Louis XVIII back on the Throne after the Hundred Days. Austria and Metternich would seem to naturally favour the Bourbons as the legitimate rulers of France, but had little faith in their ability to rule - they preferred Napoleon II but ultimately however that was not an option and they came to support Louis instead. Given Alexander actively campaigned against a Bourbon restoration it's not accurate to characterise him as an ally of that House.
I don't know the context of that 'sole protector of the German people' quote, but it's important not to confuse the Prussia of 1814/15 as the Prussia of 1871. Whilst they were German they were far more of an Eastern European power than they were German, and they preferred to gain power there rather than in Germany proper. The Congress turned Prussia into a German power almost against its will, handing it territories in the Rhineland as a direct counterbalance to France - Prussia had been an unreliable ally at best during the French Revolutionary Wars because it simply had no stake in protecting West Germany. Austrian power in Germany up until 1848 was almost uncontested, they were the hegemons, the suggestion that Austria viewed Prussia during the Congress as potentially being the 'sole protector' is incredibly suspect. Austria was not concerned about Prussian power, but you are right they were playing a balancing act - maintaining the equilibrium as Metternich would put it - between France and Russia. Metternich was incredibly concerned that they had swapped one diplomatic revolutionary in Napoleon for another, Tsar Alexander I. Much of Metternich's diplomacy during the negotiations following the fall of Napoleon were on this very point, persuading Castlereagh in Britain especially that Alexander's intentions were far more sinister than the British believed (this would finally be accomplished by Alexander's own hand when he visited Britain in 1814). Metternich realised that in order to curb Russian power he needed a strong France, and a lot of the lenience you attribute to Alexander was in fact Austria wishing to court French opinion against Russia.
I really don't know where you've come to your conclusions about Alexander's supposed staunch support for France. It was Austria and Britain that France formed an alliance with in 1815, an alliance in direct opposition to Prussia and Russian aims during the Congress. It was Britain and Austria that invited France and Talleyrand to the 'Big Four' and the Vienna negotiations.
Sources:
Adam Zamoyski, 'Rites of Peace: The Fall of Napoleon & the Congress of Vienna'
Henry Kissinger, 'A World Restored: Metternich, Castlereagh and the Problems of Peace 1812-1822'