The save-file is located at "(C:\Users\[YOUR-WINDOWS-USERNAME])\AppData\LocalLow\Wakefield Interactive\Where the Bees Make Honey".

You can access AppData a bit easier by typing "%APPDATA%" in windows search bar, but that will put you in "AppData\Roaming" so you will have to go back up to the "AppData" folder to see "LocalLow".
AppData is a hidden folder so you might have to enable show hidden folders in file explorer.

Once you find the "Where the Bees Make Honey" folder, you need to open the file "wtbmhnew.save" with a text editor; notepad or notepad++ should be good.
Make sure to close the game before editing save files, otherwise the game will get confused and possibly sad.
Inside "wtbmhnew.save" you should find that the first value stored is "levelToLoad":"[level]", where [level] can be anything; for me it was "IntroVideoSubtitled". Replace whatever is after "levelToLoad": with "officeending_old", leaving one set of quotation marks. When you next boot the game up, it should load you to the phone call at the end of the game.

Hope this helps someone. I couldn't decide what level of detail I should write this in, but it's probably good enough. If not, google can help, and to be honest I have probably spent more time writing this than I would have on a second playthrough, so it's not that bad.

Once you get both ending achievements you should have either all achievements or all but one, since in addition to them there is only one achievement you can miss. It is "Is this a dream?", and you can get it near the very beginning, after you exit the office building. Just walk straight, to the grass, and not to the wooden walkway on the left.