New World, using the concept of a FLAT earth, using a system of souls and vessels. (In need of y'all to point out the plot holes and alternatives for the same)

Hey guys, so the world I'm building is a flat earth, with two civilizations, the upper and lower (I call them the hollow). The lower civilization lives inside a dome of water, which falls off the edges of the upper flat earth, and this dome is maintained by giant root-like structures. The gravity is maintained by a dual core system, where these roots contain a core, which pushes the people of the hollow towards the land along with the centrifugal force, creating gravity.

Now, the system of souls is based around the fact that every 23 years, there's a ceremony called the "giving back," which sacrifices certain people of the hollow who are born with the mark called the "thethered.", The law of the universe states that every soul must be shared (Or is connected, and I've not thought of something that sounds logical to connect them, open to suggestions lol). The soul is shared by the upper and the people of the hollow.

The marked ones of the hollow (thetered) are born with a vessel which is larger than the soul, thereby creating a space between the soul and the vessel. Now, for the structure of the vessel to be maintained, it constantly absorbs "energy" (haven't given it a name yet) from the atmosphere for 23 years, and stores the excess in the space between the soul and vessel, the thethered use this energy as a source of "magic" or abilities.

The soul of the thethered (the marked ones) also creates a sort of vacuum, to stabilize the vessel, which is felt by the soul of the person who's sharing (or connected) the soul with the thether, making them constantly at the border of life and death. To combat that resistance, their vessel undergoes constant modification, which leads them to develop heightened physical prowess (these people in the upper have a separate name, but I haven't thought of it yet).

Now the main character was born as an anomaly; his soul wasn't shared or connected to anyone, therefore, he was born overflowing with "soul energy" (has a separate name, but I haven't thought of it yet), and he remains oblivious to this power for 22 years and uses it as a response to some pain-inducing external stimuli, (haven't thought of the trigger yet). His power is energy manipulation, btw. Since he's an anomaly, the universe constantly tries to eliminate him.

His friend, Lira, is a marked one, and he, to save her, must use his powers to kill the upper counterpart of Lira (who shares a soul with her), and transfer that soul into her, thereby stabilizing her vessel, and making her unsuitable for sacrifice. Now, by doing so, the roots, to compensate for the lack of energy from Lira, select a random person and stand them for the sacrifice, but since that person is not marked, and does not have enough energy, the roots will also sacrifice the person whose soul is connected to the substitute sacrifice, taking two lives in one go. (Though he gets to know this much later in the story)

Do keep in mind that neither civilization knows about the other's existence. (The MC doesn't know how to save his friend (love interest) from the sacrifice for most of the story, and gets to know the solution much later)

Yeah, so that's pretty much it. I know there are some fallacies, but I can't quite point them out. Open to suggestions, y'all, and thank you.