I haven't read this entire thread, so I might be repeating what other people have already said, but I don't think a more lenient Treaty of Versailles was possible with the war ending how it did. This is because, ultimately, the British and definitely the French governments wanted blood for a war that they had invested so much in. Any government that tries to be lenient would lose the next election by a lot, because they wouldn't have any support from the people.
But, while it couldn't be more lenient, the peace treaty could definitely have been even harsher, in order to truly cripple Germany and prevent it from rising into a great power again. This would likely mean either an occupation of Germany by the allies or partitioning it into smaller states.
In the short term, this would be enough for Germany to end negotiations and swear off the armistice, continuing the war and forcing the allies to march to Berlin. The allies would probably succeed in defeating Germany for good, but continuing the war just because they're too stubborn to accept something more moderate would look very bad.
In the long term, the treaty being harsher would be bad because it would be extremely unenforceable, even more so then the Treaty of Versailles IOTL. If Germany is partitioned, the only way the individual countries don't just unite back, whether de facto or de jure, would be through some stranglehold over their elections, which probably requires occupation as well. And if Germany's occupied, who's going to do it? America's definitely not. Britain might try at first, but I'd imagine that eventually, its population is going to want their brothers and sons back home. That leaves only France, which, as someone said earlier, would not be able to do something like this alone because they have a population 1.5 less than Germany's. Overall, anything harsher than Versailles would too be a failure.
So what has to happen in the war so that another one doesn't come? I would say some combination of these three things:
1. A less costly war. If the war is less costly, if it, for example, ends in 1916 instead of 1918, the allies would be less bitter at Germany, and so, more willing to entertain a more lenient peace.
2. More American leverage. If America is more involved in terms of boots on the ground, and so, has more leverage in peace negotiations, Wilson has a higher chance of successfully convincing the other two that a more harsh peace may not be the wisest idea.
3. The Allies at Berlin. If the Allies take Berlin, then there's no stab in the back theory and the German militaristic tradition is forever discredited.