Johnson, IMO, has improved out of sight. He hasn't been absolutely obliterated apart from South Africa at Perth, and that was along with everyone else against Amla, Smith and AB. Moreover, as soon as Amla was gone he completely dismantled Elgar - not that hard tbf, but the way he did it was electrifying to watch. England has Cook, who won't destroy him so much as wear him down, and Pietersen whom he can be shielded from if need be.
He could play as a bowling allrounder at 7 if Watto is unfit to bowl/outright injured. He's bowling pies? Take him off and use one of four other options. He's on fire? So much the better.
Btw, he needs his own top-line sports psychologist, not the hangers-on drawing mind-maps and parroting 'back yourself' that usually tag along with the squad.