Could not load the main_init.cfg file suggests to me that it is either a bad installation of the full conversion mod or something is wrong with the main_init.cfg file on Amadeus' side.
You have stated you have a Steam copy. Have you tried launching the game with the Steam Launch arguments which are included in the moddb description?
Due to the large and diverse amount of errors possible with FC mods, we can only provide so much troubleshooting. I'm assuming you have already, but perhaps try contacting Reminiscity on the matter, since an error preventing the game from running is likely to be caused by either their files, or a bad installation on the user's side.
I'll look into this issue since it is concerning that it has applied to multiple mods, however no relevant file change update to Amnesia has occurred for the past year, so it is unlikely something on Amnesia's side has broken.
Edit: Just downloaded and ran the Full Conversion on my end. Game parses the main_init.cfg file just fine. Therefore you likely have a bad installation; that is, you have placed the Amadeus folder in the wrong directory.
Ensure you have followed the installation instructions made available here as they are written:
http://www.moddb.com/mods/amadeus/news/i...ion-guide1
If your Steam directory is not the default one, then you simply need to drag and drop the Amadeus folder from the archive
into the Amnesia - The Dark Descent folder.
If you feel that you have dropped the folder into the correct directory but it is still running incorrectly, delete your Amadeus folder and drag and drop again, or download the mod from moddb and follow the installation instructions again.