While you are most likely right. I still have my doubts that he had made out Clean off that island. Like how much damage did he actual face from the Tailed rat mirror thing. How much was actual Wiped away clean? As if i recall it was a process that could take anywhere between a few hours to a few weeks. He didn't really know for sure as i doubt he had the opportunity to test this out. Considering how unusual Roko is as a ghost. So i don't think he knows for sure what would happen.
That said, i don't believe roko is Sucking their Mana from their body. No, that wouldn't in truth kill said creature if he did it. Least i don't recall them ever setting rules for Magic in this world. Where if you run out of mana you just cease to function. That and roko has been seen eating small animals. Which begs the question dose Everything has mana? How much mana do they have? And yada, yada, yada. No i won't lie roko to me is just eating their souls. as in the end roko at the moment is an Undead Creature. And we really don't know how undead work in this world yet.
That said did roko do it? Guess we are going to have to wait and find out. As remember Roko is a sneaky fox that likes to do stupid things. But this could also just be a red herring. So who knows where it might go.
You know, i had this theory that the Final Boss Might be Roko. Like sure the Queltz seem to be the Main Big Bad Evil guy. But outside of what the Tailed rat said. We really don't know anything about their plan for the eye. Granted i might be reading into this. But we just don't know enough about ghost in this universe. Let alone one that is a Perfect ghost like roko. As the only other ghostly figure we met was the God of death. And i don't think their reapers are that Talkative.
Anyway regardless Roko is missing with a single egg. Begs the question who would steal the egg? And what exactly is Roko doing outside of making himself the number 1 suspect?
Ps, i don't really mind Roko eating their souls or Mana. Whatever you want to call it. Everyone got to eat something. Its just brand has a soft spot for the birds. Why, i don't really know, maybe he really wish he could fly in his old body. Or maybe it is out of spite for the Queltz due to them being their ehh slaves/servants. Regardless thou considering brand is probity still mad at roko. Seems a bit inconsideration to even consider eating their young. Which if you recall page 416, Roko Defiantly was like, i want me some Baby Back Souls.
I can see him becoming a monster, though, if he eats too much mana too quickly. Think Murder Mystery (https://mysteryskulls.fandom.com/wiki/Mystery/Gallery?file=%21%3F.png) from Mystery Ben's Mystery Skulls music video series (which also features a good-aligned kitsune named, well, Mystery).