Luckily I was able to use the "restore previous version" function of Windows 7 and able to change my save file back to one at the start of chapter 4.
I then rushed right to the part where Nate & Critter are able to leave the tower, skipping any other interactions not required. So this makes a nice save for anyone else who suffers from this bug; at worst you'll have to redo a few things inside the tower, but it guarantees I won't have done anything that you might not have done yet yourself.
Here it is: https://dl.dropboxusercontent.com/u/47505238/savegame0.zip (copy this link to your browser, clicking on it in Steam doesn't seem to work)
In-game it is simply called "Save"