Couldn't Kabuto have un-summoned Itachi? I know that the Kotoamatsukami rendered Itachi free of Kabuto's control over his will. But he was still bound to the Edo Tensei, since he was released when the technique was cancelled.
 So, couldn't Kabuto simply have un-summoned Itachi?  

We have, in other situations (namely in chapter 514, with Deidara), seen that Kabuto can un-summon shinobi at will, an empty coffin appearing and 'reclaiming' the summoned. Were these other situations somewhat different from the situations with Itachi? Or did Kabuto have to completely undo the technique, even if he just wanted to un-summon a single shinobi?
The most plausible explanation is that the un-summoning with the empty coffin popping-up is only possible when the summoned is under the caster's control. Itachi broke free of Kabuto's control with the Kotoamatsukami, but he never broke free of the Edo Tensei itself (unlike what Deidara's answer suggests). This is why he could not be un-summoned (with the coffin), but was freed when the jutsu was undone.
Still, Kabuto could've undone the technique altogether, at the cost of un-summoning every shinobi he had summoned. However, this could mean a complete turnaround in the war. Anyway, Kabuto, most likely, did not think a complete release of the jutsu was needed, since was arrogant and confident that he could beat Itachi.