So... what the hell just happened?
Why did Nue have to sacrifice his mother? Why did she have to leave the girl's body?
I don't get it...

I don't think he sacrificed her, per se. She was exorcised from the girl's body thanks to Yura using her technique and Rikuo stabbing her with his sword. After that, there was no way she would have been able to remain in the living world; she just got a few moments to say her final goodbyes to her son. The good guys did exactly what they were supposed to in order to get rid of her, so that's why she was sucked into the afterlife.

I didn't quite get the reasoning behind why Seimei turned his back on her though. Was it only meant to be a symbolic gesture like he was talking about (that he'd only been able to go forward because she'd been at his back all this time), or was he also saying that he'd hardened his heart against her as well?

ETA: OK, chapters post-130 make it clear, he'd hardened his heart against her. Rikuo was pretty pissed at how coldly he treated her.

Interesting themes with mothers/parenthood being developed in these chapters. Would never have expected to see something so mature in a shonen manga. I'm liking it. 🙂