Making Schieflen-plan progress through Belgium is possible, just tricky.
Early August
First, detach at most 1 corps from GE I Army to put Antwerp under siege (a 454 corps should do it), and have the rest of GE I Army bypass Antwerp to the southeast, cutting between Antwerp and Brussels. I usually detach the 2 Cavalry corps in GE I Army to move along the coast and guard that flank. My target for the end of August for GE I Army is Lille, or the space adjacent to the north.
Second, GE II Army can move directly to Liege. You have a good chance of destroying the fort with your first artillery firing at the end of early August. Another option is to create a "siege detachment" with 1 corps + the 2 siege guns, and try to move the rest of GE II Army past Liege to the south. The detachment from GE II Army that starts the game in Germany should convert the rail line in the area just East of Liege and stay there. That rail line will be critical for supply in September and October. GE II Army is going to fall behind GE I and III a bit b/c of Liege. Don't worry about it.
Third, I move GE III Army directly to Namur, where they will usually fight a battle with 1 Belgian Corps. Set your reserves to breakthrough into Brussels, and there is a decent chance they can grab Brussels in early August.
August
GE I Army should attack into Lille. It is not a fort, so you will likely win a battle there with the British. Have your reserves set to breakthrough to the south toward Paris. The BEF tends to fight only a couple rounds and retreat, so having the reserves set to pursue is critical. In WEGO mode you have to do that at the start of the battle, before any battle rounds engage.
GE II Army has 2 courses of action: if Liege has already fallen, continue moving them south, but first drop off a detachment of the German siege arty so it can move to Antwerp to help blast the fort in October (it will take 2 turns to get there). The rest of GE II Army can move along the rail line directly south toward Sedan. The separate detachment must now convert the rail into Liege. If you can, create another detachment to convert the rail line in the area directly south of Liege. COA 2 is to leave a siege detachment in Liege with the siege guns and move the rest of the Army south.
GE III Army attacks south / southwest from Brussels, dropping off 1 corps to invest Maubuege, with the rest of the Army covering GE I Army's left flank. Their objective in September can be Reims. Note that now the order of march from West to East is: GE I / GE III / GE II, with GE II and GE III having switched.
From here, you have to decide what your goal is. If Paris is the target, send all three armies at it in September / October. Forget the flanks. If you goal is to set a solid, semi-historic line and hold it, make sure you grab both Lille and Reims (both are munitions production sites). If somehow possible, Dunkirk is a good place to anchor the line, but the Ypres area might be more feasible. If you really want to follow the historic situation, have GE I Army turn "left" just before they get to Paris and fight it out with the French east of Paris. It might work.
Some key issues:
1. Supply. Remember that supply trace is an HQ can reach back 2 spaces to a converted rail line, then throw 2 hexes away to another HQ, which can then throw supply 2 more spaces (the "middle" HQ is a "supply relay"). So, the max # of areas from a converted rail line is 6 areas (2 back, 2 out, 2 out). In addition to the rail line that runs into Liege and then south, if a GE IV Army detachment can move along the rail line from Luxembourg toward Sedan, that rail line can provide a supply connection for your "left wing" armies as well. One thing I have noticed in computer LGG is that a) you can't move and convert more than one space at a time, which is slower than the boardgame, and b) you have to move into a space to convert its rail line, so a stationary element can't convert a line where it is (very annoying, needs to be fixed in a later patch).
2. Siege Arty. Dragging these guys along will really slow down GE II Army if you let it. As soon as you can, detach the siege guns from the GE II Army HQ. I usually send the German siege gun to Antwerp to end that siege before the end of the year, and I usually end up using the other one to reduce Maubuege. Of course, the Austrian siege gun would do the trick at Paris a well
3. The French. The enemy "gets a vote" as they say, so the war plan the French adopt will heavily influence your ability to get a "historic" outcome. If the French go with Plan XVII, it reduces what they can detach and send back West to defend Paris. If they pick one of the other plans you may run into a very strong FR V Army and elements of FR I and II as well + the forming FR VI Army in Paris itself.
4. The Brits. I've played about 10 early August to October CG start ups (as the game has been patched / improved), and I've yet to see the Brits in Mons in force. Sometimes 1 corps is there at the most, with the main BEF force closer to Lille or the coast. In real life I guess the Brits had really good dice. I usually don't have trouble brushing the BEF aside with GE I and GE III both attacking it. German goal should be to push them toward the coast and screen them so they can't stay shoulder to shoulder with the French.
Hope this helps a bit. Enjoy the game!
Doug