! He was always the reincarnation of Yakou, his memories just didn't awaken until just before Natsume was killed. His plans were always some sort of world domination by the Onmyoji mages, but the fact that he went to such lengths to save Natsume proved that he was still also Harutora and that he valued her life over his world ambitions. Suzu said the ritual kind of worked without any elaboration, which could mean any number of things, but in the context of the anime, it implied that Harutora didn't die as an equivalent exchange
IIRC reading online the story was continued in the novel or manage or whatever. It was stated that she was fused into the dragon so natsume is part of the dragon now and if she uses all the power of the dragon she dies like if the ogre guy. When your tethered to a powerful being or whatever your power is like a glass/tank and there is a limit and it doesn't regen.
Living Spirit: After her death and resurrection, Natsume is now considered a Living Spirit while being bound to Hokuto. Much like Touji, she possesses the ability to access more of Hokuto's powers, but the power isn't infinite and if it runs out, she dies.
1st seal: purge - After she releases the first seal, her spiritual power increases explosively. An astonishing high-level yin aura pours from Natsume's entire body.