So Rin was a Jinchurki before the 4th Mizukage, so this means that this plan was devised by the 3rd one.
It's quite interesting that the Mist Village had a method to unseal a Jinchuriki's seal from afar.
The failure with Rin, might be the reason why Kakashi learned Sealing jutsu. After all, if he could use it at that time, then she wouldn't need to die, as long as she had enough will power to support the second seal.
Rin's body was compatible with a bijuu and she already was filled with love. Does this mean that she could have been potentially a perfect jinchuriki ?