It confirms my suspicions that the Linnea hook isn’t such a good hook to begin with.
(I'm not that keen to wade through four hours of video, so I'll take your word for it.)
If/when I run
Vaesen, I'll probably make the butler Algot Frisk the hook, as part of a "session zero". He approaches all the PCs and offers them the keys to the Castle on behalf of mysterious benefactors. A little digging will reveal Linnea (and maybe one or two other infirm or insane survivors) who believe in the Society's work but can no longer do it themselves. I might have to resist the temptation to play Frisk as
Alfred Pennyworth. (Or maybe I don't.)
EDIT (2 hrs later): Alternatively, a lawyer or his agent could approach the PCs initially with the Castle keys, then disappear into the background. This lawyer could also provide an in-game explanation for Insight Points and Upgrades: the lawyer manages a small trust for Society members to hire Personnel and outfit Facilities. The PCs must report their efforts to the lawyer, who consults with retired Society members, before he will authorize Upgrades. (Contacts emerge from solving the Mysteries themselves.)
Or maybe for the first session or two the PCs run their proto-Society out of somebody's private residence, and
then introduce Frisk, Linnea, and/or the Society as part of a Mystery. Or a PC inherits / discovers / steals a spooky old house that serves the same function. I'm all for starting
in media res, but a mysterious stranger giving away an ancient castle in the first session would make some of the players I know deeply suspicious, to the point where they'd spend more time investigating the Castle and its owners than pursuing Mysteries.