Every theory should exist on their own merit. If they’re interesting enough, they can stand alone. Most of the famous Final Fantasy theories work this way. Besides, many of these theories have very little to do with one another.
For example, consider two of the most famous Final Fantasy VIII theories:’Squall’s Dead’ and ‘Rinoa is Ultimecia’. The first one is pretty hallucinatory:
At the end of disc one, Squall and Friends face Edea on a parade float in Deling City. After the fight, when Edea seems defeated, she conjures an enormous ice shard and propels it through Squall’s chest. Squall stumbles back and falls off the platform. He sees Rinoa above, reaching to him as he falls. Squall closes his eyes and dies. The entire remaining game time, from the beginning of disc two to the second half of the ending movie, is a dream.
The second one is similarly nightmarish:
There will be an Ultimecia in the future. Her existence, persecution, and defeat are inevitable. You can’t change the past, and Ultimecia’s death in the past created the present and future. Now what if that Ultimecia was not a random person yet to be born, but one of the fighters who would perpetuate her own tragic destiny?
She being, of course, Rinoa.
On their merit, each one provides a unique interpretation of Final Fantasy VIII’s narrative. However, not only does each theory contradict the original source, they also conflict with each other. In this sense, the’Squall’s Dead’ and ‘Rinoa is Ultimecia’ theories can’t be true at the same time as they contradict each other.
Logically, two opposing ideas can’t both be true, yet they certainly can be false. In this sense, ‘Squall’s Dead’ and ‘Rinoa is Ultimecia’ are contrary to each other.
Wouldn’t this imply that one or both of these theories are true? Not if the game doesn’t mention either. Since Final Fantasy VIII states neither in the original source, ‘Squall’s Dead’ and ‘Rinoa is Ultimecia’ don’t have a basis with each other. Sure, it may seem as if they complement each other. However, if ‘Squall’s Dead’ is true, then he’s only imagining Rinoa as Ultimecia. In this world, she isn’t a sorceress and most of the narrative that might conceivably make it so doesn’t exist.
Similarly, if Rinoa is Ultimecia, at what point in Final Fantasy VIII does Squall die? The traditional answer is during Edea’s Parade Scene. But Rinoa is there. We also know that Ultimecia controls Edea from the future. Aside from Squall dying, how is it possible for any of this to be true if Rinoa is Ultimecia? In this case, Squall would have to be alive in order to carry out the story’s continuity. He would have to awake from his unconscious state after Edea’s Ice Strike. And then the story would have to at least hint that Rinoa is Ultimecia.
If Rinoa were Ultimecia, we’d need an explanation for how his subconsciousness came to this conclusion, if he were dead. The ‘Squall’s Dead’ theory provides no explanation. And none of the theories even hint to each other. They’re only based off of the source material, of which Final Fantasy VII doesn’t mention.