Up against four nations, is there really any way that Sturm could have won? The answer, I think, is a resounding "yes".
The mistake Sturm made was in trying to divide his forces and conquer all four nations at once. He relied too heavily on powerful factories that made his COs overconfident, thinking they were unstoppable with unlimited numbers. And even though Hawke nearly conquered Green Earth due to being more realistic with his forces, the ineptitude of the other COs cost him the war when the other nations intervened. When the other nations finally got to the island Black Hole had commandeered, the combined weight of their forces was too much even for Sturm's improved (since AW1) command skills.
So what could be done differently? Well, quite simply, Sturm should have only focused on one nation. He should have built all four factories in the same nation so that his forces really would be overwhelming. Then, he should have gone himself and left Lash back at the home island. (The terrain on that island is more suited toward her defensive skills.) Between the factories, Hawke, Adder, Flak, and himself, that nation wouldn't stand a chance. If he subsequently cloned the COs like he cloned Andy in AW1, this would bolster his army and make future strikes easier.
Ideally, the nation they conquer should either be Orange Star or Yellow Comet. OS is the one that organized the coalition to stop Sturm the last time, so eliminating them could, hopefully, stop another one from forming. The danger is that some of the COs could escape, and thus that coalition would happen anyway. If he conquers Yellow Comet, though, he would eliminate the most powerful nation as well as have a pair of powerful COs to clone. (Kanbei is the reason Yellow Comet's military is fear, and Sensei was called the greatest CO of all time before he retired.) Thus entrenched, Adder and Flak could return home to assist Lash while Sturm, Hawke, and the clones held the fort in Yellow Comet. The two territories would be able to hold out against assaults from the other three nations. Once they'd built back up, Hawke and Sturm could assault Green Earth and build additional factories there. Once they clone those COs, Black Hole could only lose if they somehow managed to snatch defeat from the jaws of victory.

