That's awesome! When I ran Edge of Darkness last year and was asking for advice, there was one Keeper who did a similar full-length journal for Rupert Merriweather. Those things wind up being little works of art in their own way, I love how CoC encourages these projects.
I'm always a little leery about the length of handouts given to my players, even some of the published scenario handouts I worry they break up the momentum when players have to stop & read a few pages of text. So I try to divide them into a larger amount of shorter tidbits. It's kind of a fun effect that way because it really gives the impression of accumulating clues. Journals like the one you're doing are sort of along that line too, in that each entry can be its own discrete clue - wish I had the time for something like that.
My campaign last left off in Nov 1924, so the chronology is a little off - we're going to fast forward to Sept 1925 and say the investigators agreed to meet that following year, then do a quick run-thru of what each character's been up to in the meantime. So I'll have some handouts & backstory worked out there. I have some old-school telegram & newspaper templates for that.
For example, one of our characters is a German aesthete of noble lineage - in the last scenario he inherited a manuscript of unsettling German forest folktales with grotesque engravings. In his research during the hiatus he'll discover some references to The Black Goat of the Wood & maybe legend or two about a demented fertility cult in the forested area where the manuscript originated from. This will serve as an intro to Shub and then some of the stuff in Blackwater Creek will seem eerily familiar (like you say, maybe also some hints toward the control of vermin & such).
One of our investigators is connected to the Arkham criminal world, so he'll be sent to check out the bootlegging operation as sort of a side errand. I want that to still be secondary to the search for Rhoades (i.e. that character won't be sent to actually claim control of the distillery), but at least give them some idea who the Carmodys are when/if they encounter them.
I'm going to rewrite some of the accounts of Cade's Rest and the Siacook (sp?), again maybe breaking up those handouts into a few pieces.
There will be a couple planned encounters on the way to BWC just to set the tone.
Other than the Mother's Gift mechanics (which Scott convinced me to leave intact) I wasn't committed to making any major changes. Hence asking here, I guess! I felt like the advice I'd gotten for EoD (which was my first CoC Keeping) was super helpful and directly led to me adjusting the clues for better story flow. So hopefully someone else will chime in. I'll definitely give a recap/report after our first session and thereafter.
The one other issue is that our group might be a little large to start - 6ish players. I'm going to go with that even if it's not my preference, with the assumption that scheduling conflicts will probably claim a couple of those. But if we *do* have that many investigators I'll increase some of the hazards, plus make a point of them being a very noticeable group.
There's the potential for quite a bit of gunfighting (Carmodys and sheriff both), which I have mixed feelings about. At the same time, the isolation & hostility of the town is sort of a character in its own right so that danger is pretty apt.