When Belkar attacks Roy with Twin Daggers of Doom, he's attacking from Range 1, not Range 0, so Roy cannot use Greenhilt Sword. After the attack is resolved, Belkar moves into the room and can take a loot from Roy.
I'll have to go back to look exactly how it's worded, but I think that if at the end of your turn you're in a room which you cleared out the last monster, you can take 2 loot. Consider that to be the case unless I come back and edit this post.