I’d try undoing the changes you made, as they seem to be what broke the game. If that doesn’t work, try a clean install.
I bought the steam version of thief 3 and installed it along with the sneaky 1.1.10 full edition. Adjusted a few minor things with the sneaky tweaker, then ran the game. It played fine. Played it a few more times on the first mission (getting caught over and over by the guards in the hotel), and then opened the sneaky tweaker to change some more minor things, and tried running the game again. Now the game just shuts off within the first second of the title screen. Any ideas on what happened?
I’d try undoing the changes you made, as they seem to be what broke the game. If that doesn’t work, try a clean install.
It just worked by itself somehow. Ran it again to see if anything worked by chance and its working.