French mobilization was actually faster than the German, and they concentrated along the border with Germany, so "strike the French faster than they can mobilize" doesn't work.
This is not to say that forgoing the invasion of Belgium, but still concentrating in the West is a bad idea. Its valid, but its at first glance it is the worst of the three options available:
1. Strike through Belgium, either against Paris or the French rear, as they tried historically.
2. Defensive in the West and try to get something going in the East (objectives would be limited).
3. Go through the fortresses along the French-German border.
Now #2 actually was considered, but you are committing to fighting a long war, which the German leadership didn't think they could fight, due to the problems of importing critical war materials. I've never read that #3 was given much consideration.
It helps to look at where the armies were after mobilization. Armies varied in size, but if you just assume ten divisions per army you are not far off:
FRENCH GERMAN BORDER
3 German (5,6,7) vs. 4 French (1,2,3,4)
ARDENNES/ LUXEMBOURG
3 German (3,4,5) vs. 1 French (5)
LOWER RHINE
2 German (1,2). This was their strike force. When they went into Belgium they faced the Belgium army and the BEF
EAST PRUSSIA
1 German (8) vs. 2 Russian (1,2) with another one on the way
GALICIA/ RUTHENIA
3 Austro-Hungarian (1,3,4) vs. 4 Russian (3,4,5,8) with at least one more on the way
SERBIA/ elsewhere
The Austro-Hungarians planned to concentrate what amounted to 2 and a half armies against the Serbs, the 2nd, the 5th, and the 6th, basically the Bosnia garrison, being the half. In the event they routed the 2nd to Galicia, too late to take part in the opening campaigns.
So what gives the flexibility in the German plans are the 1st and 2nd Armies, which are their largest. They don't necessarily have to use them against Belgium. The Belgian army isn't going anywhere, and the BEF won't necessarily be committed right away otherwise. They should be employed where they can do the most good. Parts of the group of the 3,4, and 5 group can be added to this, but at least one of these has to be kept on the Belgian border in case the French try something.
Logistically, its feasible for the Germans to rail two corps to completely different front. They had doubts they could do this in August 1914, but in the event did this during the war several times.
Are hammering against a narrow fortified front, against the biggest opposing concentration of armies, really the best option for the 1st and 2nd Armies? It would seem even doing nothing with them and keeping them in reserve would have been a better option.