So the message I was after was in fact a bit misleading. The DLL could not be loaded as I expected, but not because it would be invalid, but rather because it depended on some missing libraries. One tool that's great for such things is the Dependency Walker. It'd tell you whether the DLL is corrupted or missing dependencies. It doesn't know where to get the missing DLLs, but at least it tells you the dependencies are the problem and the names of the DLLs.

P.S.: If you reply to my post, I get notified.