If you free him before all the leaders are dealt with, he tells you that he can't leave before they are and asks for your help. You can then tell him to sit tight until you have dealt with the leaders or ask him to come along and help. It all doesn't sound much like a plan, he got captured while wanting to get something else out of the trip and now he refuses to leave until the leaders are dealt with. There is nothing in the dialogue that suggests he had some grand plan how to take them down, he just sits in his cell until the pointy pebbles hurt too much.
As for him breaking out when you defend the Grove but don't kill the other leaders: I haven't checked if they survive in such a scenario but I assume it since Ketheric has dialogue about having talked to Ragzlin about the adventuring party.
