You misconstrue, the punishment room in Arctis Tor is the NeverNever backdoor to Demonreach, but you have to get past Mab AND then Alfred in the real world. But in doing so it provided an idea of the force required to overcome Alfred with a Warden, so not a total failure. A first attempt at a break in to ascertain how bad the defences were, for a better organised attempt in the future.
They successfully did the former and failed on the latter. For most beings Demonreach is a one way ticket, word of Alfred is very unlikely to get out. Only a handful of Wardens may have actually been privy to its full secrets with nothing ever written down because of the Archive, and if originally it was only accessed by the NeverNever, (likely as most of its history predates midern transport) its physical location may not have been known, especially as it doesn’t appear on maps. This made it more difficult for Kemmler to get back to Demonreach. Mab had closed the backdoor and made travel to the island by boat dangerous, especially for a wizard whose power is grounded by magic. The connection between Merlin and Mab in Peace Talks has been established
listen to the Wind and McCoy are relatively ‘local’ so may have figured out the rough area, getting final directions from Harry, merely confirmed it. As Blackstaff Eb may have been the last to deliver a Prisoner to Demonreach before Harry, presumably via the Never Never. Fortier may have been taken to the island by Rashid in the Never, Never with the blessing of Mab to become Warden and never went back.
Mab clearly had met Kemmler, hers is a personal opinion, which tends to support this theory. Rashid would shepherd a new Warden via Arctis Tor and probably make an introduction to Mab. This is pre Accords, so Mab would have been less on speaking terms with the Senior Council and other Wizards at this point, except Rashid.
Much of the series since Grave Peril has been about (1) controlling Harry and (2) controlling Demonreach with (1) and (2) becoming the same thing in Turn Coat.