Is France going to invade Germany through Belgium in this TL? Else, I certainly don't see how exactly they could quickly reach the Rhine; after all, in this TL, the Franco-German front line is going to be much shorter and thus much easier to defend than it was in our TL.
Most likely they do or at least through Luxembourg. Even if they don't, the Germans will have to assume that they might and send forces to cover the frontier. In any event the Germans simply don't have the troops to meet the French and send four armies East. The Germans use four armies to defend Alsace and the Ardennes. It leaves nothing to defend against the French 5th Army, the Army of Lorraine, the Army of Alsace or the Army of the Alps- none of whom are committed OTL.
there are just a whole host of problems for the Germans-
the German iron deposits are all West of their fortress line. If they fall back to it (the only logical thing to do if your on the defensive) you've already lost the war.
they suffer 300,000 casualties OTL. That's more than enough to decimate an army of 800,000.
The French can make good their losses (the French don't commit their reserves initially while the Germans do),
the French can strip their fortresses of artillery
the Germans can only commit three armies to the East, the fourth army has to wait for the rails to be freed
The Germans had many competent generals. Not one thought of going East. That should tell you how bad it is. If that doesn't convince you, consider that the Germans preferred going to war with Belgium and Britain rather than going East Moltke was clear: there is no East first plan and you cant leave an undefeated France in the rear