Here's another question: can the Confederates send any additional reinforcements to Georgia than they did IOTL? IOTL, Grant seems to have expected Early's troops (which he estimated at 20,000 or 25,000) to be sent to Georgia after they had withdrawn back to Virginia following the raid on Washington. At least, he thought it was a strong enough possibility to send Sherman a telegram warning him to expect considerable Rebel reinforcements in mid-to-late July.
Assuming the Confederates felt reasonably secure in the Eastern Theater for the time being, could they have sent, say, two divisions of troops to help Johnston in mid-to-late July, as Grant feared? They had been able to do so the year before, but the situation was obviously different in 1864.