TBH, to me mel's mother seems increasingly less relevant. One on hand there is virtually no chance of the demon king ever having felt any love for this individual. And with that in mind, there is zero chance of the demon king having allowed this hypothetical person a chance at forming a relationship with her sons.
So far we know that originally there was only chaos... and from chaos sprung the DK and SD and then the sacred tree. Then each god went on to create their respective races. Meaning that the DK and SD actually created at least the ancestors of their current living race members. We have pretty strong evidence that the gods can effectively create life at least.
Now.... This makes it perfectly viable for the gods to have created meliodas, zeldris and elizabeth and thus they don't have a parent. Now, I would venture the guess that at least when it comes to meliodas and zeldris the DK must have created them special in some way since they were meant to be vessels and are in some meaningful manner their children. My guesses to that guess are that meliodas and zeldris were either made out of the DK's power, kinda like the original demon, or perhaps they were made directly from the DK's body. As in from a rib he tore out or something.