Hanna levelling up her Copy skill from the XP/SP she got from the treant (it's not like she was doing anything else during the MC's fight) then using Copy on Yanagi again would have worked just fine. There wouldn't be any need for Rin to beg Yanagi to switch to Hanna, or a dumb rule about Plunder going to whomever kills anyone who has it. Hanna's copy would be an entirely new instance of the skill, and wouldn't be affected by Yanagi still having it linked to Rin.