Even if they had conquered Rhineland, which I think It could, your big mistake is to count people of Rhineland as french men.
France population remains under 40 million people whatever territories It occupies.
France's big mistake in 1914 was that It had not enough understood that it was heavy artillery that mattered. France thought what would be decisive was the bravery of its soldiers. It was not sounder 25 years earlier.
The french army became superior only when It made a technological, strategic and tactical breakthrough : the use of tanks at the serviçe of a mobile strategy in 1918.
Before this, thinking they could have won a war against Germany is an illusion. If It had been a reality, they would have had their revenge at the time.
The question here is can the French between 1971 and 1914 ever be able to accept war with Germany and have a reasonable chance of success. The answer is clearly "yes". From about the late 1880's up until about 1912 the French actually are likely to win such a war.
Even in 1912, the germans are not so superior that they can be assured of victory- in fact, its only the idiocy of Joffre that gives them any chance at all.
The Germans launch their invasion with about 1,3000,000 men. Against this the French have-
1,000,000 in the Regular Army
450,000 in the reserves
180,000 in the territorials
200,000 in the GVC guarding communications
800,000 in the fortresses
Only a truly incompetent idiot (i.e. Joffre) can manage defeat when attacked by a force of less than half the size in a war that favors the defenders. Even Joffre has opportunities to win the war by, for example, successfully exploiting the gap between 1st and 2nd army during the Marne
Yes, Germany had advantages in heavy industry and manpower but doesn't exploit that advantage until after the 2nd Moroccan Crises leads to abandoning the Naval Race. Before that, the Germans lack many of the material advantages of the 1914 campaign and even a Joffre should be able to manage a successful defense of France
The bottom line- if the Germans tried the Schleiffen Plan in 1905- when the Russians are basically out, they are going to be defeated. You can subtract the ten Italian divisions from Schleiffen's numbers because they aren't joining. Then the Germans are going up against the Belgian and French northern fortresses without the heavy artillery advantage that they used in 1914 and Schleiffen is bringing in the Dutch against the Germans as well.
All the Germans are going to achieve is bloody losses as they batter themselves against the French fortresses in battles that resemble the siege of Port Arthur. Eventually the Germans are going to tire of it and the French can launch counter attacks against a weakened, bloodied and demoralized German army. The French are very likely to get to the Rhine and then the war ends unless the Germans aren't able to get their armies out of the Rhineland in which case the French are going to have a field day invading Germany
However, once the Rhine is reached, the French are likely to be satisfied having achieved any reasonable objective. That some of the conquered people are Germans is irrelevant. They can be taxed and contribute to France's defense and the Germans lose them so they will be much poorer. In any event, the Rhine is a beautiful defensive line