asangas: I encountered the "_INMM.DLL was not found" error and missing serial key error, after doing the offline install. The serial key displayed in the downloads section for the game on the Gog website came up as invalid when entered into the game.

I managed to get fix it by following these exact steps:

1. Installed the game from GoG Galaxy.
2. Downloaded Offine version of the game using the "extras" tab on the game in Gog Galaxy, and installed on top of the existing install (no uinstall was done first).
3. In GoG Galaxy app, under more options for the game, select Manual installation -> Verify / Repair. This patched the exe to have the correct licensce key.

I haven't yet tried doing step 2 first, followed by step 3, skipping step 1 all together. But that could potentially also work.