I'm heartbroken but I somehow don't want seiran to be resurrected, I'll choose to have this as bittersweet memory rather than having her come back. I'm sure things won't be as great as before
I don't think it is easy to reanimation a person. Mimi's mom is a powerful witch with special heal ability. Even that she need to exchange her life. If not, school will try to create as many Mimi as possible already.