I completely understand why they might want no one in charge. With no one in charge, the island can't defend itself as well because there's no wizard directing or improving its defense but no one can easily release the prisoners either. Harry could release them all if he wanted to. I think that the island is just as safe overall because Harry makes it harder to break prisoners out using an army but easier to break them out using subterfuge, blackmail, or similar. Everyone knows that Harry would commit genocide to save his kid so they could reasonably guess that he'd release a prisoner to save her too. If the Formor really want Ethniu back, they have to know that kidnapping Maggie and demanding Harry to release might actually work.
I don't know if it is that simple.. Actually when Harry did in the Red Court it wasn't just to save his daughter, it was to save himself and his grandfather as well. What he did was merely reversing the spell that the Red King wanted to use to destroy him and his grandfather by killing little Maggie.. What the Red King hadn't counted on was Harry figuring it out and that Maggie's mother Susan would be willing to sacrifice herself as she was becoming the youngest of the Red Court vamps to save her daughter.
In chapter 39 of Turn Coat, Rashid has closely questioned Harry about what he knows about the island. Why it is there, it's purpose, etc., it becomes very clear that Harry is totally ignorant of any of that. All Harry knows is it's affect on people/beings and thinks it is a way to level the playing field for someone who he thinks is innocent, that is Morgan. Rashid also explains that the island is the source of the powerful leyline. However though he says it is a useful question, Rashid doesn't answer that question. We don't know if he knows or not.
On page 299, Harry reveals to Rashid that he has made it his sanctum. Rashid asks him how? He further tells Harry that he doesn't know what he has done, and that he will pay a price for what he has done.
Then he gives Harry some advice; bolding mine
"First," he said, "do not tap into the power of this place's well. You are years away from being able to handle such a thing without being altered by it."
The important bit here I think are the last words in the sentence,
"without being altered by it." Which implies to me that others have tried and
were altered,or corrupted by it, which could very well have happened to Kemmler, he tried, wasn't ready, and was corrupted and altered by it.
Then on 379 in his journal Eb writes that it is quite clear that Harry had no clue what the real purpose of the island is. Then more interestingly;
I sometimes can't help but think that there is such a thing as fate--or at least a higher power of some sort, attempting to arrange events in our favor despite everything we, in our ignorance, do to thwart it.
This to me suggests that Eb does think that the island needs a Warden, that up until that time the Council has pulled back from appointing one either out of ignorance, or fear.. Fear perhaps that by appointing another Warden, they would be creating another Kemmler.
He goes on to say what Rashid said that he didn't think warning Harry about the dangers of the island would do any good. Eb says that Rashid is a good judge of people, but that he isn't sure he is right. Is Eb saying that Rashid is saying Harry can handle the job? Or is Eb afraid that Harry cannot? Eb says he trusts Harry's judgement, goes on to say that he is one of three or four wizards who could handle that mantle.. But then he goes on to doubt himself, saying he trusts Harry's judgement, but then he also trusted Maggie. Not so sure about him really trusting his daughter though, I don't think he did,