Makoto effectively cut off Nebula's head which then fell into the chest. I think it's sitting atop the core itself. The tiny sphere the Idolmaster sits in is way in the back at the very top of each head. You can see the top of the sphere Iori is in when Makoto and Hiems blow the spike off. There's no connection from the head to the core anymore.

Looking back to the preview, which Idol is Yukiho in? Either way, whichever it is, it's not happy. Chihaya is gonna be rejected by Imber again...

I can totally see some keyless Haruka-Imber fighting action.