Responses
You actually cooked so hard because
You're right! Sheria is the actual Snake God. Kibrin's past life was as a god-killer and was supposed to kill the Snake God, but they fell in love. The Snake God ended up being killed when they were separated, and he purposefully reincarnated with her so that he could act as a scapegoat and keep her identity a secret.

Its not spoilers since i dont know the story but....
I think Cheria is Del, and she was the last princess of the Moritiya kingdom and the snakrs mate. Thats why Kibrin doesnt thunk fondly of Rose without the seal but maybe he has some leftover scent of Del because they were mates. I also think Del was kidnapped by some other being (which is pretending to be god now, who the temple reveres) to get tovthe snake, and that caused the downfall of the Moritiya kingdom