This is probably more suited for the crazy theory(?) thread but...whatever.
One other thing I could see being the case would be Merlin being the bastard child of a goddess and a demon. She might've lived with the goddesses as a child/young adult but had neither clan's inherent powers and was treated like trash, sort of like a squib in Harry Potter maybe XD, she fell in love with a goddess clan member but got rejected as someone who isn't of pure blood (thus her most embarrassing memory) and she probably also tried to research her second nature as a demon which got the head honchos to shit on her some more.
She eventually meets Mel and Liz 1.0, decides to flee the back and forth between the two big clans and learned to use conventional magic to an excessive degree, maybe killing someone important (to her) in the process of her magical research.
It'd fit the generally more tragic backgrounds of the other sins, it might explain her long live (can goddesses die of age?) and it'd definitely explain how she met Meliodas, who's a reject himself since he betrayed the demons.