Well, Leipzig was decidedly the decisive battle. He had most of his troops there and most of the Russians, Swedes and Prussians too.
First of all, this is a defensive battle where the attackers have encirclement and troop count going for them. This means they will probably not leave until either the commanders are dead - which almost happened and could allow for a white peace or a peace at the price of Poland - or they've taken massive damage either in terms of casualties or morale.
Therefore, these are the conditions for a French victory at Leipzig.
Massive morale damage means the French can pursue and inflict additional casualties. Of course, France will have to make concessions, since 1812 had proved the Russian Bear was too much for France to invade, but they basically have the edge.
There is a problem in that.
France at that point as very few cavalrymen, the reason Napoleon failed to crush the Allies in Spring was because he couldn't pursue because he lacked cavalry, so in Leipzig, even if he win and I admit that possibility, he will only be able to repel the attack and, if for some reason, he decides to pursue the enemy he will fail because of lack of cavalry to inflict serious damage to the enemy.
Massive casualties means the Coalition troops in Germany lose their numerical edge, France can recover its Elbe defense lines (Leipzig is in Saxony, therefore not so far from the Elbe). There too, France can get a correct peace.
The "massive casualties" you speak off would be at max seventy thousand out of a Coalition Army of 400,000 and the Coalition could accept that while the french couldn't lose 30,000 as they did OTL.
Going to the Elbe is impossible at this point. If he going further into Germany he is forcing bigger supply lines and with the Peninsular forces advancing against France he can't allow himself to keep his army in Germany with France in danger he will have to go back to the Rhine.
Of course not, he cannot impose anything, however with France in a position of relative strength, it might be possible to preserve what's left of French power through skillful negotiation (e.g. if he gets Talleyrand on the job)
He didn't trusted Talleyrand at this point and Talleyrand would accept terms Napoleon wound't. If he wanted Peace he would had sent Talleyrand to meet with Metternich so that the Armistice of Pläswitz could become a real peace.
Again Leipzig is the decisive battle. Winning it strengthens the French position because it means the Coalition cannot use its numerical advantage for a while.
To Leipzig to become decisive to Napoleon he as to be the one to encircle the enemy and not the other way around. Leipzig is only decisive to one side, because it allowed them to win the war more quickly, but for Napoleon it would only be a time delayer and a bargain chip during negations.
Let's assume Napoleon has come to terms with reality - all of Europe is up against a France which has lost its elite troops. France is already going to lose a lot of its military enthusiasm, since a whole generation of young men have been expended on the battlefields of Europe. In this context, Napoleon would not be able to declare war again if all the other powers of Europe remained in a defensive alliance.
Unfortunately the other powers would always see him as a threat and would demand that the French Army suffered restrictions on it's size and number of guns they could have.
Napoleon II can accept this, aka his regency can accept this, but Napoleon I would never.
Also, I repeat, at the time of Leipzig, Austria is still deliberately neutral. It only really joined the coalition after the French defeat at Leipzig. That's why the battle is also decisive in diplomatic terms.
For a neutral power to commit the entire army of Bohemia, some 115,000, at Leipzig and to suffer the greatest amount of casualties 34,000 it surely is amazing.
I tremble in fear of what would had happened if Austria (you know it's people, Emperor, aristocracy and army officers) was hostile to France.
So, let me recapitulate.
The English want France out of Belgium and Rhenania. Sure, but if their allies in East Europe make peace, they're screwed and won't count in the negotiations, unless they occupy Belgium and Rhenania.
The Brits are paying for the Coalition if they cut the money the other ones are easy pray to France and they know that, they will never go behind GB back because of that.
The Prussians want France out of Rhenania. They are a minor power, which is very vulnerable to losing its capital again after the Battle of Leipzig.
Russia may want France out of Rhenania, but what they mostly want is France unable to attack them again, and additional buffer lands. This means Poland is prioritary over the French Rhinelands.
A little problem is that the minor power as eighty thousand angry and vengeance seeking men at arms and that the Russian Emperor had pro missed to aid them in their demands.
Prussia and Russia are on bed at this for the better of for the worst.
Both agree that Poland must seize to exist, again.
On the Rhineland, Prussia wants it's lands back and according to the Treaties of Reichenbach the Coalition will protect and aid them on their claim to them, also a French Rhineland threatens Prussia and by existence his ally Russia.
As for Italy, it's mainly an Anglo-Austrian concern. Since the Austrians aren't in the war yet, they could be easily bought into joining back the French over some concessions in Italy, which would bring balance back into the Coalition War. Typically, returning the Illyrian Provinces to Austria and the Latium to the Papal state would be quite big concessions for France, which wouldn't have to go all the way to dropping Piemonte and Liguria.
Why do you say, again, that Austria wasn't at the war at that point?
The Treaties of Reichenbach signed on the June 27 made the Austrians commit themselves to the Coalition if Napoleon refused the peace offer. In Leipzig the peace offer was long refused and Austria is part of the Coalition.
Catalonia would be a matter of France with the UK and Spain.
I think the Spanish would accept getting rid of the rebellious Catalonians if they could get rid of the Bonaparte on their throne and get their rightful King back.
It would create an uproar in England if the BEF happened to be squashed by the French army, which would certainly go in favor of making peace
Spain accepting to get rid of Catalonia????!!!! They still haven't refused the idea to annex Portugal, and by that time we were independent for a very long time, and do you think they will just say "Sure just keep it. Oh, by the way do you want the Basque and Galicians? They are always making us troubles."
Jesus Christ mate but Ferdinand would never accept that. The poor excuse for a King would demand all of Spain, not part, all of it.
My conclusion is :
France could keep the Rhine Frontier and its Transalpine provinces, as well as possibly Catalonia, if it's capable of winning Leipzig, bring the Austrians back on the Napoleonic side with an Illyrian bribe, make peace with the Russo-Prussians (and the Swedes), and then squish the Ibero-British army in 1814.
That is your conclusion, and I respect that, but I disagree on it and have defend the why on the points above.
In the Spring Napoleon can win the War, but by Leipzig he as lost too many men and gear.