This is one I've often wondered about. By November 1918, the German Army was beaten but by no means routed. The withdrawal through Belgium and out of France was still fairly orderly.
History tells us that newly revolutionary societies can be both militarily aggressive and successful - look at France from 1792 onward and Bolshevik Russia in 1919. Both were able to withstand powerful internal and external counter-revolutionary forces and I suspect that had the alliers entered Germany, the newly republican Germans would have resisted fiercely.
Indeed, would not the Germans have in any case have seen a foreign army as an invader rather than a liberator? We know in 1944-45 there was fanatical resistance in both east and west by the Germans (much more so into 1945 in the east but look at the fight the allies had to secure Aachen and the Hurtigen Forest in late 1944).
So, let's imagine a lesser uprising with a more combative Imperial effort to put down the mutinies and disorder while at the some time continuing the war in the west.
November-December 1918 sees a patchwork of confused fighting within Germany as British, French and American forces advance slowly but steadily from the west encountering strong resistance in some areas but almost none in others.
Bavaria declares independence but soon fragments into internal conflict as does Austria and most of the regions of the former Habburg dominion.
The onset of winter further slows the allied advance but on December 11th 1918 the French reached the Rhine at Bonn, south of Cologne. Hastily-assembled German artillery barred the way to the east but the German Army was collapsing under internal tensions, food shortages and mass desertions.
By mid-January, reconnaissance showed German forces dissipating east of the Rhine. On January 15th, under an artillery barrage, French and British forces advanced across the Rhine bridges while American troops improvised a waterborne crossing near Oppenheim .
The French and British soon advanced east as the Germans collapsed in front of them. They found towns full of starving people desperate for food and fuel.
I think this is pretty realistic.
The peace will be harsher in some accounts - more territorial losses, Germany likely will be partitioned into several smaller entities, maybe the leadership is put to court, the whole fleet will go, military restrictions will be much harder. Considering reparations, I can imagine that it will be somewhat lighter: everybody by then knows that Germany cannot afford much, and the US influence will have grown massively during the later months of the war.