Plotting Cosmic Dread / Designing a Trail of Cthulhu adventure

As an intellectual exercise I've started writing up a Trail of Cthulhu story idea I've had.
It immediately went through two revisions as I realised it was too complicated and over ambitious, and pared it down to the core concepts.
I really want to do something with monsters, rum-runners and prohibition, so my original idea was to have the players be Feds and investigate some liquor smuggling on the Canadian border, get stranded in a remote hotel during a snowstorm and get caught up in a play off between the Mi-Go and the Cult of Hastur.
It sounded great, but I struggled with working out how to introduce the separate plot elements.
My usual downfall as a GM is to be too ambitious, and then fail to deliver, so it really makes sense to reduce the concept to a simple premise and develop that. I can always include other bits as I go on.
Remember, you can always add, but you can never take away.

So, with this in mind, I threw out the Cult of Hastur, and focused entirely upon the Mi-Go. In a further effort to simplify, and to try to keep the game as Purist as possible, I decided to cleave the structure of the story as close to the structure of The Whisperer in Darkness as possible, although told from the more immediately horrific perspective of Henry Akeley.

This seemed to exclude the originally intended setting of a secluded hotel, so that's been replaced with a Rangers Station instead. More Wrong Turn than The Shining.

So, with this revised story in mind - Federal Agents, whilst investigating a distillary in the Vermont hills, are trapped at a Rangers Station during a blizzard along with some Rum-Runners, and are menaced by the Mi-Go, who are mining nearby.

The next challenge, the one I specifically set myself, was to write a ToC Gumshoe adventure, with Core Clues, Investigative Ability matrices and a coherent narrative thread that leads from the initial set up through to the Horrible Truth.

After a failed attempt to organise my thoughts on paper, I've now turned to a flow chart app on my phone; Thinking Space.
Whilst its still a work in progress, my first 'mind map' is below.

From this base I intend to tie together the intended events, locations and characters with a network of clues.

How this will translate to an actual game is yet to be seen, as the last time I used a flow chart when plotting a game, the results were fairly pedestrian, and involved a fair bit of rail roading, which I'm against.