>>49391370
At least with the arm, her dialogue initially is specifically "as I thought, I should be a proper oni and devour you, flesh and all." None of it ever really comes off as personal. Obviously full body Kasen wasn't a proper complete reunion, as there hadn't been the time for it to finish, but Kasen definitely didn't give the arm full reign to succeed with the sealing. Hell, it seems more like she'd given up entirely after the trick was revealed, and although we don't see the rest of the battle, nothing afterwards indicates that Kasen ever fought back for control.
The other issue is that, because Suika is alive, the only example in the actual myths I can think of that would have something like the Arm's situation (Shuten Douji's severed head trying to kill Yorimitsu) wouldn't be relevant, and Tsuna obviously wouldn't have been pulling his punches to "just" seal Kasen. It seems to me that the Arm containing part of her "self" was a deliberate ploy to survive and eventually fully recover, and armless Kasen's personality was an unforeseen consequence of that. In the same way that Suika's ability to gather people is probably why she was able to actually lead an organization of Oni when according to the fairy mangas they typically don't last. At the very least, despite Oni being the most straightforward and forthright youkai per PMiSS, they've got some of the most subtle lore details.