I wondering, when Mimi supposedly "hurt"her head, did Shiina subconsciously use her healing magic, the sparks that arose from her seemed like the same on from the plant.
I feel like when the doctor (Furan Sensei) said she was irreplaceable it was foreshadowing and irony that she's going to be Shiina's precessor and is going to have to look after Mimi after she's gone.