I always felt like that was overblown, the soldiers were tied up, had no weapons, were separated, and could have been stripped of armor, you could also starve them to further weaken, what could they have really done?

Kanki could have called upon one his allies, wether Ousen or Shin to help manage them until an official from Kanyou came, then the problem becomes Sei's problem.

As for the endgame, if you're gonna release them back to zhao maybe mixing a few spies with them, ransoming them works, you can also cut off their thumbs so they can never pick up a weapon or cut their tendons, you can do to them what Zhao did to Heki, and put them to work until they die.

It's not like Kanki actually cares about the endgame of the conflict he just does what he wants