The reason Lafortier was killed was to open up a seat on the Senior Council. That is stated explicitly in the text. Could he have been an absentee Warden? Sure. But why? That's almost as bad a not having a Warden at all. To communicate with Alfred Harry has to go to the island. Did LaFortier have a yacht in the harbor at Chicago? For that matter where did Kemmler park his canoe?
For Christos? He seems only to have been a stalking horse, we have seen nothing to suggest that he was more than an politically ambitious individual fed up with certain aspects of Senior Council politics. He wasn’t Black Council he wouldn’t have been part of the White Council delegation in Peace Talks otherwise. Remember everyone (except Harry) was suggestible at this point. The Black Council gained nothing by elevating Christos, so why murder Fortier? If the murder the Warden and suggest that it was internal politics it throws off the pursuit.
The point is It was known in the White Council from Small Favour Harry was interested in the island, enough to panic.
There is a way onto the island Rashid can use it, as he pleases and Maeve was able to use it once to bring Peabody and the spiders. This suggests that the way comes out deep in Winter, most likely Arctis Tor, and I think we have already seen where, a room where Mab keeps beings prisoner in blocks of crystal ice, where they can reflect upon their misdeeds. Sounds familiar?
The attack on Arctis Tor in Proven Guilty may have been an attempt on Demonreach by Nemesis and the Black Council. Just because you know where something is via the Ways does not map onto its location in the real world, and Demonreach hides its location. A way from Arctis Tor to Demonreach may long have been known and used in the past, but subsequently guarded by Mab. That attempt failed because Fortier was Warden and doubtless the Alfred was set up to guard it in his absence.
This mean’t identifying the Warden, killing him and taking up the Wardenship themselves. This attack would have been set up in Grave Peril giving Lea the Athame, and follows on from the Nemesis strategy of dual attack on the Outer Gates and Demonreach. If Cowl had been successful in Dead Beat he would have taken down the well protected White Council headquarters, in the process killing the then unidentified Warden.
Maeve and the Black Council may have only discovered its real world location in Turn Coat, if it is something passed down orally amongst the Senior Council and Blackstaff and the Warden (the Archive didn’t know) and then not to all members of the Senior Council. Spooky island may not have equated with Supernatural Supermax for most of the Senior Council.
This allowed for a direct attack in Cold Days, and Nemesis trying to access again by deceit in Battleground. Thrice denied. Four if you count Dead Beat.