He is de facto incredibly powerful, but he is de jure not. He's only capable of doing those things because the checks and balances failed through the coordinated efforts of hundreds of Republican congressmen. Every heinous act that goes unchecked bears the implicit seal of approval of the entire Republican party. He could do many of the things you claim he's powerless to do, too, but he doesn't want to, and all the mechanisms meant to force him to have failed.