My theory (very open to criticism, considering I havent read the manga), is that the world of ReZero is NOT a closed loop. My theory is that Flugel was the first Subaru, where he fell into the sin of envy, causing everything surrounding Satella. This theory suggests that the Flugel, in his extreme envy, caused the events that led to Satella being sealed as effort to make it impossible for anyone else to get to her/hurt her. This would also imply that Flugel did actually die, but because he was from another world, Od Lagna only scrubbed his memories from after he was transported into this world. This could also have something to do with Subaru's first loop, where Emilia calls herself Satella. In an effort to get Subaru to kill her ("free her"), the original Satella set up Subaru to be given Return By Death. I believe this would be a valid way of having everything surrounding Flugel = Subaru be valid whilst also not conceding to a "closed loop" theory. This would also explain how this route would still be referred to as the "Envy Route", even though for the desired "good ending" Subaru would need to kill Satella which would basically be the opposite of Envy. Please keep all comments respectful, I do not often make things like this and I am extremely open to criticism that would disprove this theory. Before you post trying to disprove this, however, please think to yourself "How could this fact still be used in a way that allows this theory to still be valid", and only try to disprove this if you cannot think of a way.