"Japan, Italy, and... France?"
Two players will be USSR in co-op mode.
Fortresses have been toned down alot. Even level 10 to the atlantic isn't a gamestopper for an invasion.
Yes, it does. But fortresses only take a few months after the tech and they take on the order of a year beforehand. So you still don't want to build forts beforehand. Its just not worth it.
Focus on infantry. If you want to do the whole fortress line, its 5IC each for at least 4 provinces - 20IC. Budget when you're going to build your troops to allow your full manpower to be turned into infantry by the time you expect a war - decide when he may attack you. If Germany's worried about your preparations, he may attack in march 1939, for example. He'll have most of his helpful events and he can even get the M-R pact while at war if he doesn't annex Poland before it fires.
If you are hot about the fortress line approach, I would suggest a line of Le Havre-Paris-Compiegne-Rheims-Maginot. You lose 8IC, but its after your major builds anyway. This line gives you a better defensive position with the rivers, reduces his org and separates fast and slow units further by having him take more land, and it opens him up to an amphibious assault behind his lines. He'll have fewer troops available and you'll be better entrenched and on better terrain.