The safe seemed to be at a high level in the building.
9th is a very clever bomber.
She could have used those skills to destroy the building in a way the safe falls quickly enough to kill eleventh when it crashes on the ground.

We still do not know who Chibi is.
We have yet to know why God did that strange thing to 9th
Regarding Yuno, 2 of them have existed at the same time at some point.
We do not know who the current God is, could be Yuno or Akise.
After all they are god, so the strange thing in the chair could be there to deceive everyone.