My guess is that the "Goddess" is an existence similar to the Demon King. Likely created around the same time, no doubt from the original Demon King's lover vessel.
That would explain why the Hero and heroine constantly fail. The one who is actively preventing the destruction of the remnant of the original Demon King is his very first love.
She likely polluted the original king's mind. Played things around the second time and I wouldn't be surprised in the least if she *was* the woman in the Heroine 3rd reincarnation, or at the very least influenced to what level she went.
How's that sound?
And this time she is possibly putting all the pieces(/people) together to reunite with her lover via some unholy "holy" ritual that is supposed to deal with the Demon King true vessel this time around.
Meanwhile the Demon King, wise on her antics, is trying to prevent just that. He is not the original demon lord. He is the hero that consumed the Demon King's soul. His feeling to the Heroine are stronger than his faded feelings to the Goddess that likely feel like distant memory of another person's life more than anything else.
This the goddess doesn't want. She wants her original hubby back. So she is playing the pieces in such a fashion that it will likely play out that way (not knowing that the current Hero is wise on her plans)