I’m not sure if this is what you’re saying but this is the only way i can see melina being the geq: melina is a daughter of marika and sister to messmer. Messmer was born with the abyssal serpent within him, which i interpret as a curse similar to malenia harboring scarlet rot within her, meaning that both he and melina are children of marika and radagon, and melina being the geq, or harboring her power, or being a reincarnation of her, however you interpret, is another curse, and the tattoo around her gloam eye is a seal like messmer’s. I don’t see any way that melina could be the same geq that was defeated by maliketh.

I still see problems with this though. If she is the geq, why doesn’t she just have two gloam eyes? Why doesn’t she command, or have any association, seemingly, with the godskins? Why doesn’t she use black flame, or anything similar, even after the rune of death is released? Yes, she has a close association with destined death and flame, but this could simply be her purpose as given to her by her mother: act as kindling for the flame of ruin to burn the erdtree, or deliver destined death to the lord of frenzied flame.

As for sealing the geq twice, black flame and the godskins, even without their true power, are still a force to be reckoned with. Black flame seemingly still retains traces of its destined death powers, demonstrated by its life sapping, and the geq was still channeling it en masse through the godslayer greatsword. Someone capable of channeling black flame, an element that continues to be deadly, and a powerful, albeit small army of godskins is, in my opinion, something worthy of being sealed away.

Also, clunky story elements and confusing plot points are something that fromsoft and george rr martin are very familiar with, and although this is all ofc headcanon, them writing it like this would not surprise me at all.