This guide will help you to savescum the save file so you can resume a chapter after ending the game, as the game makes you start from the begging. Plus to that, I'll explain how to preserve your stats ingame (ends unlocked, chapters found and decisions made).

This guide is based on CrazyCanuckJ's guide of getting all achievs in only 2 runs (~3 hours).

First of all, you want to know when you want to savescum the game. As suggested in the CrazyCanuckJ's guide, I would do that in the 10th chapter, as from there, you can go to pretty much any ending.

After getting to the desired chapter, you want to go to the directory:
and copy the "GameData" file to another folder (desktop, for instance).

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The next step is only necessary if you want to keep the stats updated with your progress.

After you get your achievement, the "Your decisions" tab should have been updated with the new stats. To keep these stats, you need to open the original GameData file (in the game directory) with a text editor and copy the "<Ints>" section with its 3 lines, which should look like this:
After that, paste it in the backuped file, as show bellow:
Please note that the right file is the original and the left one is the backup.

And now, both files should have the same "<Ints>" section, looking like this:

Now, all you've to do is to replace the original GameData file with the backup one. Please, make sure that the game is closed before doing it.

After replacing, open the game and click resume. If you do it right, the game should resume in the chapter you savescumed, while the stats are updated to the last completed game.