As we talked on discord with Flyoutfate, I tested it on another computer with a save that was created on the faulty PC and got the same bug. I tried with a virtual machine but can't launch the game in good condition due to lack of ressource on my computer (I got 1 image per 4-5 secondes with graphism and resolution at the minimum). Edit : Oh and I tried to uninstall/reinstall, begin a new save from scratch, etc...

To explain what I did to copy my save, it was simple, I only copied the DDDA.sav to have a speedrun save and on normal. After that, I couldn't kill the hydra in first hit like I did before. I don't know if the file 0 to 7 in the save folder have an impact. And finally, it was 2 years ago so I don't remember well, I maybe forgot to deactivate the steam cloud save so it could have broke my save, I dunno.

I think it's not possible, I try to load a save from a friend and the game thank it was corrupt.

And for my confort, I tried to get one save for the speedrun and on save with all my stuff and it seems to bug the game. In the hydra fight in the encampment , I can't one shot the hydra, I need to wait after it destroy the tower before kill it, so about 45s time lost.

You should complete the game and farm all the item yourself to speedrun the game, it's the better and safer option I think.