My first thought was "Trenchcoat Demon" for the three-people-in-a-robe creature but I suppose that wouldn't quite fit the setting (but maybe acceptable if its existence is intended to be humorous).

A thing to keep in mind is that localization teams may or may not have access to the creators and supplementary information, and some of the choices to emphasize a different aspect of the creature or translate less literally may be a result of behind the scenes conversation.

EDIT: Also author seems to be not quite familiar with standard Japanese fantasy terms. "Dragon zombie" might sound striking when you first hear it, but to a Japanese player, that's just what undead dragon enemies are called, most of the time.

>But I still have one question. If Gwyndolin inherited the power of the Moon and therefore was brought up as a daughter, why wasn't Gwynevere, a full on Sun heiress, brought up as a son?..

there were two enormous reasons why that wouldn't have worked