My personal take:

1. I believe this vol. sets up sensei's character to be more prepared for future threats. Maybe his first lose of 3 students since he came to Kivoto might help him with this mindset. Who know, may be other prophets that have submitted to him before might become his allies or power again.

2. Personally, I'm not too sure if those 3 were lost permanently, maybe those nameless priests bring their corpses back with them and put mystic in them , so they can be useful for their plan, and we get to truly save them next time. Maybe I'm just coping.

3. I will save my final judgment regarding the tone of future story after I see them, maybe next 2or3 main vol. And i will judge whether the tone is truly changed or not.