Technically what they're doing isn't that different to what Mahito used to do, they're just trying to keep the shape of the soul, so any mistake could be disastrous. But also they mentioned before about how invasive the act is and is being done without consent, so even if it goes well it's not an honourable thing to be doing.

Yeah, it feels like we missed a chapter where Marulu stops Dabura's fight, presumable by helping which is why he was allowed access to the black rope, and at the same time grabbed Yuji into the soul plane.