I love thinking exercises like this, but this one really degenerates quickly. Everything the space marines "take" is held by IG. All the forge worlds, all the supply lines, everything but the front line is held by IG. Without a foundation to stand on. Without those millions of people behind the space marines, they've got no firm foundation to fight a war. Take that away and the space marines are just glorified tin cans. I mean do space marines know how to fly ships? could they pilot their own craft? I suppose they can but still when they arn't it's held by IG. Space marines entrust EVERYTHING else to the IG so in an all out war they've got nothing.
So if you just have a big battle all the IG vs all the space marines all at once in one giant cluster fuck? Well IG will probably win. Space marines can do some amazing things, but in a fight like that, it doesn't play well to their strengths.
In a longer more drawn out war they run out of supplies. Sure they can take large amonts of what they need from IG, but that basicly means they have a scavenger existance. Eventually IG would stop making stuff compatible for the Space Marines to take, and then their supplies run dry and now you've got space marines with Flashlights trying to fight the IG. They will lose eventually. Unless your telling me that Space Marines know how to repair that ridiculously huge armor they have on.
BUT here is the worst part of this whole scenario. IF the Space marines did win... what then? They don't know how to be farmers, builders, workers. All they know is war. Bloody lot of good that is going to do them if they don't have those millions of people to fuel those absurdly huge fleets. Even if Space marines could win, they still lose.