I ended up diving through some old chapters, and I noticed there was some foreshadowing to the Winged Lion's interest in Marcille. Back when Laius first meets the Lion and sees a vision of his dream kingdom, Marcille ends up being the main administrator in Laius's vision and the Winged Lion is impressed by her skills. He even says that she'd make a good dungeon. It seems like he picked Laius first because of his skill and desire, realized that Laius is a little too weird, and pivoted to Marcille.
Also, looking back at the chapter on Marcille's nightmare, she says her dad died early and her mom told her that she would live longer than most people- was her mom an elf and her dad a human or some other species with less of a life span? She says that she grew up near the island instead of near elf lands, so that corroborates that theory.