I just have a few questions about this theory:
You refer to the Guardian serveral times in relation to Lavos being knowledgable about the future. We can assume that the Guardian was not constructed after 1999. The Guardian was probably made before 1999, thus giving Lavos complete knowledge of it without having to know the future... Would this even matter to your theory?
And about Lavos existing in his pocket dimension during the Day of Lavos, yet people could see it from the outside, yet only looking through to its dimension. Doesn't the game imply that it must have been physically present in the world during that time? I mean, we do see the Earth's crust start cracking, and then Lavos burst through it and shells the planet with God knows what to destroy it. If Lavos simply existed in his pocket dimension during this time, why would the game show him bursting from the ground, which would have required his physical presence to do so?