The Nightmare challenge on the eighth floor of the Illusion Realm was indeed where Yino had arranged to meet Yuebai.
Yino hadn't planned to have Yuebai shadow Xumo the whole time. Xumo was already suspicious of Yuebai, and as they progressed through each floor collecting artifacts, the power gap between Xumo and Yuebai would gradually shrink. Eventually, Yuebai's illusion techniques might not fool Xumo's eyes anymore.
So Yino had only tasked Yuebai with following Xumo to the fourth floor. Once she'd confirmed Xumo's chosen path through the dream trials, Yuebai could slip away and meet Yino at the predetermined eighth-floor Nightmare realm.
This way, as long as Yino could track Xumo's first four choices, he could roughly predict Xumo's final Dream Speech outcome in the Illusion Realm.
