Decoder.exe file is as you guessed is out of process decoder, if one of them hang up for example Similarity can't do anything with it, it just attaches itself to new one and skips scanned files, old one can still in memory hanged up. This scheme gives us huge benefits, just imaging which would be if hang up one of threads of Similarity.exe on middle of scaning. To find problematic file you can see logs located "%appdata%\Similarity\logs" folder (enter in Explorer).
Similarity doesn't specially process link files it depends on file system and OS.
