My only question about the whole thing is... why would the WC separate itself from DR? Going from a single Warden, with all the power/potential DR represents, to multiple wardens of far lesser potential (though greater range of influence/activity) seems like a very important step. I can hardly image what exigencies prompted them to it.
This one seems pretty easy to me. We know that DR has a very strong negative effect on anyone who steps onto the island. And it’s almost instant. If the WC is as paranoid about wizards going warlock, they would most likely not want a powerful wizard camping out on that island for any length of time, since the island’s presence would eventually drive em insane and probably turn em warlock – a very powerful warlock since this wouldn’t be the sort of position you give to apprentices.
“But,” you say, “Harry gets along just fine with it.” Yes he does, but he did something no one else ever did – he performed a sanctum invocation there. That’s what allows him to sleep peacefully on the island and not dream of very bad things. Rashid was positively dumbfounded that Harry would try something like that on the island. His shocked astonishment says to me that no one ever tried it before, therefore it is not something any of the prior Wardens did. If such a procedure was the standard first task of a new Warden, Rashid would have reacted much differently (more along the lines of “wow – figured that one out all on your own did you?”). His shock that it was even possible speaks volumes.
My guess is that it was a vacant role for a very long time since the White Council either couldn’t get anyone to accept the role, or the ones that did went full-Sith in a hurry. But I’m also guessing that one was able to hide his warlockry from the WC and function as Warden for a chunk of time. I base this on the ruins of the town. The tower was obviously put in place as part of the original construction. The town, however, is much more recent. It takes considerable time and manpower to build a town, even a small industrial town. All of the people required to build it, maintain it, and such, would have been on the island, for a while. Michael Carpenter, as insulated from badness as he is, can barely stand to set foot on the island, so how would a bunch of laborers, carpenters, plumbers, manager types, fishermen, etc be able to stay on the island as long as was necessary to construct the town? The only way I can see that happening is a former DR Warden went all spooky side, but managed to hide it from the WC. He was able to enthrall a large group of mortals to come to the island to set up his little fiefdom, but they eventually succumbed to the competing pressures of their thralldom and DRs “go away field” and turned psychotically insane. The WC would have had to show up, execute the Warden for being a warlock, and with the rest of the surviving population hopelessly insane, probably mercy killed them as well. If Michael is so affected, it seems impossible that a bunch of vanilla mortals would have been able to settle on the island long enough to get an industry up and running without help. I can’t think of another situation where that many people would be able to hold out on the island for that long.