at least when considering the worlds we saw so far, he did not get weaker when entering a shadow world. also he still haven't "heard" the mission (if some chapter there he does tho, such theory becomes almost inevitable).
the only issue with such "dangerous" theory is that it isn't "dangerous". it is only a rabbit role of conspiracies. like including time travel freely or multiverses. so if this world is a shadow world that triggers shadow worlds, who could tell that his last world wasn't also a shadow world. everything is shadow world. everyone are a shadow memories. everything doesn't matter. there is no way out. like our own rabbit role about the simulation within simulation (...) conspiracy/theory.
also, does the author want to make everything this sad? he will slowly protect "his people" at least some, but eventually lose everything... i don't think desir mind will hold up. i wouldn't. it is such a heartbreaking filled plot line.
What if the world Desir is currently in isn’t the real world or what he thought is the past. What if it is a shadow world he crossed into right before his death and that shadow world is about the fall of the world itself like what happened at the fall of the state with that williem guy. Crazy right?