I found that in the game, it was easier to win as France when I actually fought for my allies, i.e. deploying some troops in Belgium (if I play the historical alliance) or in Czecoslovakia and Yugoslavia (if I go with the Little Entente) to ensure they survive and stay in the fight.
Almost every advice I found on the net told me to turtle behind forts, but that means I'm fighting alone against 2 major powers, one of them far superior. Every time I tired that, the line ended up breaking at some point after a few month, and I was defeated. But when I fought agressively, defended my allies, took the fight to the germans, I had a real fighting chance, and usually end up winning the war.
Those neutral countries are not just useless color blobs on the map : They're troops, divisions, industry, planes, airfields, defensible positions, manpower, stuff you want on your side rather than obliterated or controled by the axis. They force Germany to fight on several fronts, overextending their lines. And if they fall, they might take a big chunk of the Brit army with them, further weakening the allies.
Manpower is an issue, but not as strong as I thought. First, because Maginot means a fair portion of the line can be held by a few smaller, weaker colonial brigades, leaving my real combat troops for more important fronts. Belgium and the Netherlands have smaller armies, but back them up with 24 French divisions positionned along a natural defensive position, and they can defend their countries for a long time, time enough for you to bring more troops to the frontlines. And if you really need more manpower before the war, you can always go on the decolonisation path and require troops from your fresh new vassals.
As for the idea of extending Maginot to the sea, that doesn't sound like a hot idea. A good fort needs to buid up on natural defenses. In a plain, "fighting from a fort" just means the enemy will go around your position, isolate you, and kill you at his leasure.
As a reference, the eastern perimeter of Paris is surrounded by an old line of forts dating back from the 19th century (due to our... "rich and long?" common history with Germany), and nearly every one of them was built to take advantage of geographic features : at the tip of a plateau, on a major bridge, or closing the gap between 2 rivers. And on places where natural defensive positions are rare (the plains and old marshes at the north-east of Paris), the fort network is very dense, with 1000-2000m between 2 forts, and smaller redoubts between them. That kind of fortification density would be unattainable on the belgian border. And we're talking about the kind of defense density people thought was necessary BEFORE heavy artillery, mobile warfare and warplanes made them very expensive practice targets...
