At this point I'm kind of lost and borderline annoyed. Just too many characters and places being thrown into a box and shook up.
All I can fathom at this point, is that Kelvin has caused this whole situation unintentionally through his over the top battle lust.
So my guess is
Goddess and Kelvin get engaged to marry
They kill the Demon Lord on World A which was unsatisfying so Kelvin wanted to fight his fiancé.
Goddess can't give it her all because love so by some means she splits herself and send Kelvin to World B
The bad goddess half preps a grandiose adventure and then proceeds to become the very Demon King Kelvin wished to fight
Meanwhile the good half is none the wiser and just plays the goddess role again however with far less competence because I'm guessing some of her critical thinking went with her other side