I just got an error when trying to load a chapter 3 save (more relevantly, one from last september). I'm guessing it's the one you mentioned in the development log. Out of curiosity, since I occasionally like messing with renpy stuff, I thought I'd look at the traceback and see if I could figure out the error. To summarize, it looks like you reverted to Ren'Py version 7, but the chapter 3 saves expect version 8.
Now, I just tried copying version 8 into the game folder (taken from another game since the engine is just copy-pasted when building) to see if that works. It does. However, without knowing why the version was reverted, I'm hesitant about this solution. Is there something about version 8 that wasn't working?
If not, then the solution that players can do requires only 2 steps: 1. copy and replace the "lib" and "renpy" folders from a game using version 8, then 2. copy the other game's executable and rename it to match this game.