I have read your plot synopsis. The only non human race you mention is the manaketes, how do the others fit in? And the four demigod girls, what are they doing now? Are they working to help bring back their father? Being worshipped by humans, and working to keep their father in the past where he belongs? Or a combo of the two with each one having their own views? Who was the mother, and what about her made this god fall for her? Was she a warrior priestess or something? Think about how Byleth's mom is described, very briefly but we can gather from what Jeralt says about her, why she was so special and why he loved her.
I think your non human races using the human weapons would be an effective masquerade deal. Think of Rhea using magic until she's really pissed and takes dragon form. Something like that.