Wouldn't belgium say just no like OTL?
In fact, there's plenty of evidence that the Belgians would have said "no."
More to the point, though, Joffre is handcuffed from going into Belgium. Again, I direct Aphrodite's attention to the Terence Holmes article
I excerpted earlier in the thread.
If Moltke had followed Schlieffen’s real intentions for the counter-offensive conduct of a two-front war, the first great battle of 1914 would have been fought in Lorraine in the third week of hostilities, on terms much more favourable to Germany than they were at the battle of the Marne. We can reconstruct this alternative scenario because we know exactly what the French chief of staff Joseph Joffre intended to do if the Germans did not invade Belgium.
French war planning was constrained by two political imperatives. In the first place, France was committed by agreement with her Russian ally to launch an ‘all-out and immediate’ attack against Germany as soon as possible after the outbreak of war. Moreover, the French government had resolved not to encroach on Belgian territory unless the Germans did so first. Joffre was therefore obliged to incorporate in his war plans a variant which allowed for a full-scale offensive avoiding Belgian territory altogether, and that would have come into effect in 1914 if the Germans had stayed on the defensive and not entered Belgium. For this eventuality Joffre decided that three of his five armies, comprising some 60 percent of his first-line troops, should invade Lorraine on 14 August, aiming initially to reach the line of the river Saar between Sarrebourg and Saarbrücken (Doughty 2010, 146-8, 155-8, 168). Ominously, that position was flanked at both ends by the German fortresses of Metz and Strasbourg.
Schlieffen had long before outlined how the Germans should exploit a massive French incursion through ‘the relatively narrow space between Metz and Strasbourg’. The aim must not be to push the enemy back to his fortified border. Rather, he had to be engaged on three sides, ‘from Metz, from the Saar and from Strasbourg’, and brought to a standstill there, which would give the Germans an excellent chance of decisive victory by means of envelopment attacks out of Metz and Strasbourg. The ultimate aim of this ‘attack on the enemy’s flank and rear’ would be to surround the French invasion forces and ‘not just defeat them, but lay them low and as far as possible annihilate them’ (Boetticher 1933, 260).