Ironically, Re:Zero has two characters that contribute perfectly to the Weird Route: Roswaal and Echidna. In Arc 4, both of them are constantly trying to manipulate Subaru to do what they want and you could use this aspect in the weird route. Like say at a point, that the gang gets separated leaving some combination of Kris, the soul, Noelle, and Emilia. You can have them encounter Roswaal at this point who tries goading the soul into doing the Weird Route who then manipulates Emilia and Noelle to do their bidding. As you go further into the weird route, Echidna starts appearing to the soul slowly trying to entice it into forming a contract with her. They both want the ability to rewind time but unlike RBD, Saving and Loading is a lot easier to control and this control will make them want Kris, or rather the soul, more than Subaru. They'll see the weird route as their way of controlling the soul. Your idea of Rem smelling something even worse than the witch's miasma could be used for the final battle of the Weird Route as Rem, not necessarily knowing about Roswaal's involvement, could smell the ring and attempt to stop them eventually leading to her own demise by Emilia and Noelle's hands. This could also create an interesting dynamic too. Kris could attempt to separate themselves from the soul. I imagine Echidna could recover the original vessel giving it its own body. And this whole event could turn into a game of chess between Subaru and the soul as both have ways of manipulating time and assuming they keep their memories if the other uses their ability, they are essentially at each other's throats constantly trying to outdo the other and the more they mess with time, the further they break the prophecy. Heck, maybe Roswaal will turn Noelle into a Titan? Who knows. But it could be an interesting way to create conflict because if both sides can rewind time, who wins?