And yet the Mantid Paragons pretty much allied with him. Mind you, they didn't try to take the Heart back from Garrosh because he was abusing its power or damaging the remains of Y'shaarj in any way, they defended him from our assault. Once you've considered that the Mantid clearly didn't give a shit about Garrosh but were fervently loyal to the Old Gods, Garrosh's conviction to be the one "in control" becomes rather meaningless.
That's because he was doing what the Old God wanted. Y'shaarj, or whatever remained of it, wanted Garrosh to win, it tempted the orc's pride by saying that everything should have bowed before him and further motivated him by showing Garrosh the results of his "glorious destiny". Y'shaarj even mocked us by saying that we couldn't defeat Garrosh and our cause was hopeless. Honestly, there is absolutely no difference compared to the way Ner'zhul manipulated Arthas. And yes, before he picked Frostmourne and became a literal puppet, he was "in control" too.He does show you the visions he did to break you, but he himself has not changed in modus operandi
We have a canon statement from the devs saying that Garrosh's True Horde would have reduced Azeroth into an hellish new Outland if they won.What it would have lead to had he won, which was supremely unlikely given the forces arrayed against him we have no idea, so speculating is pretty pointless