I agree, thematics do NOT trump the rules. Another example of a debate I encountered on the same day as the Khorayt "shadow" debate was about gold and search cards. The heroes were under the impression that search items were allowed to be carried into new quests if not used. I showed them the rules and the section about search cards going back into the search deck at the end of each quest. They then argued that if not used, they should get double the gold. For example, they would get the 25 gold up front for finding a stamina potion, and then if they didn't use the stamina potion, they could sell it back for ANOTHER 25 gold, basically getting 50 gold for 1 stamina potion. I tried telling them that this was incorrect, but they were already mad that they couldn't keep their search items between quests so they weren't having any of my reasoning. I can't find anywhere in the rules where it says that their interpretation is correct, but then again I can't find anywhere where it specifically says they're wrong either. In my opinion, therein lies the problem with this game. It's far too open to interpretation and can easily be taken over by stronger personality types.
The game isn't that open to interpretation. In no game ever can you make the argument "well the rules don't say I can't do that". The rules don't say I can't declare myself the winner if you don't want to get punched.
I don't think the problem is the game, it's the alpha gaming players. They are insisting how the game should work, with no regard for how it affects you or the balance of the game. They are pushing you around, even if they are your good friends in other settings. It's really just a subtle form of bullying. I'd have to stop playing games with these people.