Game runs, yet is completely unplayable

[Mudbill]

Well, this issue is rather unique. Don't think I've stumbled upon anything quite like it before myself. If I had, I would at least have something to reference as a possible solution. I think Frictional Games is (unfortunately) very busy these days with tech support for SOMA, as that's their priority right now since it was released just last week after 5 years in development. I highly doubt they're ignoring this, I just think they haven't seen it yet.

Though, did you get the game on Steam? If so, a refund should be possible within 2 weeks/2 hours gameplay I think, though you might be beyond that. Edit: My bad, you surely are beyond.

I really wish I could provide more useful information.

If you do have it on Steam, perhaps it's worth trying to disable the Steam Overlay. You could also try posting the hpl.log file contents here. The file is found in Documents\Amnesia\Main. It might give some hints.