I think folks are overlooking something, here.
Wizards love secrets. "It's like crack cocaine for them" is IIRC an exact WoJ quote.
Jim has also stated (paraphrased this time) that the Senior Council has all the information to easily figure out the shadowy "Black Council" plot against them, but that much of that info is snippets in different wizards' heads; they are keeping so many secrets that they cannot see the forest for their own personal trees.
Now, consider one Harry Dresden: wizard, snarkmeister, and generally disrespectful jerk (from the WC pov) and ALSO possibly / likely (depending on the wizard making the call) a warlock.
I'm pretty sure the Gatekeeper knows quite a lot about the island & the prison. I bet LTW does, too. I am pretty sure the Senior Council, as a whole, does not know very much about the island; just that it exists and is a ley-line nexus of dark energy. That's enough to make it dangerous.
Because if they did, the prospect of a possible/probable warlock in charge of it wouldn't be Defcon4, it'd be Defcon5 and an all-out effort to kill Dresden.
Remember how many of the SC came to Demonreach when Harry arranged his "parlor scene" / confrontation? On the island, the Warden is a credible threat against Mab herself. Those councilors -- senior as they are -- would fall like ninepins. If they knew what Demonreach was, what it could do, they wouldn't have put themselves there when they had such doubts about Harry.
QED: the "senior council" -- as a body -- doesn't know what the island really is, what it can really do. Only a few of the individual wizards have any idea... and they're busy smoking their own crack-pipes, not sharing with the others.
(n.b. they don't have to really understand the island to know it as a major locus of dark energy, which -- if claimed by a dark wizard like Kemmler, as a sanctum -- could vastly magnify power. They barely managed to take him down, with an entire ocean between the island and the warden! It's obvious he'd be unassailable in any major power-sanctum. Exact knowledge not needed.)