As for the matter of merperson origins, I find the idea that there's no evidence whatsoever rather hard to believe. The evidence concerning human origins may suffer from some glaring omissions, but we've deduced quite a bit from what's there. As for your mythology, I suspect that would be as useless to understanding the real world as the human old wives' tales about merpeople having an obsession with human souls.
2/2, and I feel that final sentence merits an apology...