Below is a collection of save files -- anyone can download from Dropbox without an account.

You can replace your local saves with these files and get each missable achievement with a few clicks.

This is scummy, but I don't feel bad about it. Getting most of these achievements is very tedious and (IMHO) adds nothing to the narrative or to the experience of the game. If you feel scummy, don't use them.

and I don't know if they would work on mac or linux. They might (they're just JSON files) and you could try it, but no promises.

On PC, the savegame directory can be found at:
You're going to want to save a backup of all your files before swapping my files in or following any portion of this Guide..

For most of the achievements, you can leave most files in ^this directory undisturbed and merely swap in a single "EP" file for the relevant episode. (If that doesn't work, leave a comment and we'll play with it to figure it out.)

For the collectathon achievements that span multiple chapters ("The Whole Story" and "Playwright") you'll need to swap in multiple files. See Instructions in the relevant portion of the Guide below. I think you can always leave your ACHIEVEMENTS file in place (but make a backup before proceeding anyway).

The game's internal files count episodes starting from the first Season. So:

https://www.dropbox.com/s/mrbx5qmljq1xa6e/EP5.json?dl=0

• Swap in this save file, then launch the game and "continue" Episode 1.
  
• Walk to the right and examine the mirror.
  
• If the achievement doesn't pop up, leave the room.

https://www.dropbox.com/s/rtysc4vw36zhq66/EP5.json?dl=0

• Swap in this save file, then launch the game and "continue" Episode 1.
  
• Walk to the left side of the screen.
  
• Enter the combination on the safe: 6-1-5-4

https://www.dropbox.com/s/k8kqpsh8bgcm6q7/EP6.json?dl=0

• Swap in this save file, then launch the game and "continue" Episode 2.
  
• Use the lever on the music box, then set the dials (see link to Fox!'s Guide just below) and click through the cutscene.
  
• Follow the instructions that Fox! provided in their Guide:

  Originally posted by Fox!:

  You need to visit every location possible in the hypnosis session. These are the commands for the shortest route:
  

  1. Go elsewhere - Wait here until an additional dialogue option appears then take it.
     
  2. Walk towards the fog
     
  3. Go back
     
  4. Go West
     
  5. Go West
     
  6. Go back
     
  7. Go North
     
  8. Go North
     
  9. Go back
     
  10. Go East
      
  11. Go East
      
  12. Go South
      
  13. Go back
      
  14. Go North - After this you will only have one option, follow it to the end.

https://www.dropbox.com/s/844cpyqphc05bkt/EP6.json?dl=0

• Swap in this save file, launch the game, and "continue" Episode 2.
  
• Inspect the skeleton.

https://www.dropbox.com/s/ugo3v7a3hvmm21d/EP7.json?dl=0

• Swap in this file, launch the game, and "continue" Episode 3.
  
• Exit right.
  
• Examine the burning wicker man.

https://www.dropbox.com/s/cgow0yhye1jjx16/EP8.json?dl=0

• Swap in this file, launch the game, and "continue" Episode 4.
  
• Use the piano key in your inventory on the piano.
  
• Put the music sheet on the piano.
  
• Play the sequence without errors. See instructions/image in SkyDivingL's Guide.
  
• If you make a single error, you will need to repeat these steps, replace the file again, and try again.

https://www.dropbox.com/s/z5ctqxx6sg9fj4r/EP8.json?dl=0

I forget what screen I saved on:

• Swap in this file, launch the game, and "continue" Episode 4.
• Try putting on the mask, if nothing happens walk right 1 screen then do it again.
  
• Exit to the left to walk down to the stage.
  
• Click through dialogue.
  
• Devitt's Fate: walk behind the curtain before it closes.
  
• Wakefield's Fate: let the curtain close.

For this achievement, you'll need to replace ALL your local save files with the ones I've put on dropbox. (Technically I think you might only need STATS and EP6, but let's play it safe.)
https://www.dropbox.com/s/mmsbpwhbgs9ykmz/20211212_EP6_WHOLESTORY.zip?dl=0

• Download this zip file, unzip it, and swap all these files into the savegame directory.
  
• Launch the game, and "continue" Episode 2.
  
• Inspect the book on the floor, and the achievement should pop up.

https://www.dropbox.com/s/tx1mkauaqa2watg/20211212_EP8_PLAYWRIGHT.zip?dl=0
I'm assuming you already have the Neophyte achievement for finding one secret on your own. If not, then try this first:

• Download this zip file, unzip it, and swap in only the EP8.json file to your savegame directory.
  
• Launch the game, and "continue" Episode 4.
  
• Play one of the secret sequences on the Piano (See image below)
  
• Exit the screen.
  
• After that, follow the instructions below, and replace all files including EP8 again, to get Playwright.

For the Playwright achievement, you'll need to replace ALL your local save files with the ones I've put on dropbox. (Technically I think you only need STATS and EP8, but let's play it safe.)

• Download this zip file, unzip it, and place all files in the savegame directory.
  
• Launch the game, and "continue" Episode 4.
  
• Play one of the secret sequences on the Piano.
  
• Exit the Screen if it doesn't pop.

Here's an image showing the keypresses for one of the secret sequences (Monkey Island theme):

• I found Fox!'s 100% achievement guide to be the most helpful for pinning down what was required for the "The Whole Story" and "Playwright" achievements.
  
  
• I found SkyDivingL's walkthrough to be most useful for many of the other achievements.

Thanks to them both.

If you haven't ever seen it, this is the insane Forum thread where tons of dedicated people took months to work out all the Secrets in the game. They're awesome.