Well, but Austria, the UK and France signed a secret alliance at the beginning of January 1815 that was clearly aimed against the Russian and Prussian interests on Poland and Saxony. This might have been a bluff, but if say Tsar Alexander reacted badly to it and decided to "call the bluff", I don't think that France and the UK could afford not to honour the treaty. Their commitment could very well be half hearted tough, and the war might and after just a couple of battles. If Austria is defeated on the field I don't see the other two fighting "to the end" for the sake of the Wettins and of "balance of power".
What could the peace terms be in the likely scenario of a limited Russo-Prissian victory?
Did they really sign a secret treaty? There is little doubt that Talleyrand was willing to support Austrian positions, but there is quite a large gap between this and committing by treaty. Castlereagh is even more unlikely to undertake any formal commitment, even a secret one, and anyway it's doubtful that his brief contemplated such an eventuality: I'd love to see him before parliament explaining how he went to Vienna to sign a lasting peace for Europe and came back after having committed to support Austria in a possible war scenario. Vienna was a very unusual diplomatic congress, since there were not formal meetings but rather informal discussions among the 4+1 and a lot of informal diplomacy: the suspension of the works at the end of 1814 was very convenient for every one of the Powers, since it provided a breathing space and the opportunity to continue the exploration of possible compromises to defuse the Polish-Saxon crisis.
What happens when the congress meets again in the spring of 1815 depends a lot on how much these attempts are successful, and obviously on how much the czar wants to push. There is also another major wild card in the pack: Napoleon. The escape from Elba was not a spur of the moment decision, but was planned carefully enough and based on the expectation of Bonaparte having a ready power base in France: the potential risk must have been known to the Royal regime too, and IMHO it would have been a powerful incentive not to get involved in a war.
The Napoleonic wild card can certainly be taken out of the pack (either he manages better his suicide attempt or is simply intercepted by the British when trying to escape from Elba) but what actually happens in TTL would be heavily influence by what happens to Napoleon.
Let's say that Napoleon is not a danger, and the Congress meets again on schedule without any external distraction: the most likely outcome is that a compromise will be found to give Prussia all of Saxony and Russia the full kingdom of Poland. Something will be found as a face saving sop for Austria: the most likely place to find it is in Italy where there are some pieces of real estate to carve out (Parma for example, as well as the Papal States) but I would expect that Metternich will want something in Germany to counterbalance a bit the Prussian expansion. There will be a game of musical chairs to be set up, since both FA of Saxony and Charles of Borbone Parma will have to be compensated somehow.
The second possibility is that Metternich stalls for time: it might lead to Prussian encroachments in Saxony, and the threat of a fait accompli. If it gets so far, Austria will have to cave: the compromise will be less palatable for them.
Finally it might get to a war, but it would be really an outlier. Best guess is that Prussia and Russia will concentrate on Austria, while the British and the French make disapproval noises but refrain from committing. A couple of lost battles and the Austrians will have to see the light. I don't envision a punitive peace treaty, but the Austrians will get a slap on the wrist of two. Prussia getting the presidency of the German Confederation for example. Russia gaining some more of Austrian Poland. There might be some repercussions in Italy too, but I would expect them to be mostly related to the game of musical chairs I was mentioning earlier (partitioning the Papal States). Murat should be able to keep the throne of Naples if he does not do anything stupid, but it would be a very strange world if Eugene got Milan.
As far as the British and the French are concerned, Hanover might get something and the French reparations might be reduced.