Boy, Isayama did a great job on this chapter.
I was wondering last chapter how everyone was going to team up when everyone had a grudge against the other side. Though to be honest I want Eren‘s plan to work.
To be honest, when Genya heard Muzan's voice I was worried Muzan would do something to Genya as punishment. Now I'm thinking Muzan can regenerate him. After all, Muzan gave his blood to Kokushibo and Genya did eat parts of him.
Your theory makes sense, Cobra's words and actions during the Reverie arc were death flags.
Although I do not think Sabo's dead, he might have been captured but dead? Doubt it.