Why the Trigger had to be used on the top of Death Peak is a good question. It obviously has something to do with Lavos, considering it resides on the top of Death Peak. Nothing is really mentioned in the game as to why Death Peak is the key. I assume it involves Lavos's energy and the fact that it is reproducing on the mountain. A great amount of energy would be used to make new Lavos's. Maybe the Chrono Trigger uses that energy to create a time freeze at the last moment of the deceased life, using the clone as a reference to the person's genetic makeup.
Hum this have been asked a while ago in thsi treath but I'll tell my opinion, death peak is where Lavos "exploded" and reproduce while destroying all the humans domes, so this would be where Lavos would finally exit his pocket dimesion. The same palce where Crono was killed and since the pocket dimension is supposely perpendicular to the timeline it could be accessed to anytime...
Do the games ever mention the entity besides that one scene, and in that one scene do they mention that the entity would be at rest when Lavos is destroyed?
The entity is mention to the forest aprt and at the very end (doN,t know which ending) but it's never told it was at rest only that it didn't need of the time portals so it would surely closed them all (which would make epoch useless, since he used those to time-travel?)
Belstasar said "us." Remmber, he was talking to Serge, and us would have been him and Serge, who certainly arn't evil. Then he says unwanted humans.
I think us would mean the Human race, but since considering the planet-entity let's say 90% or + are using the planet without giving it back nothing, so the "us" was maybe a huge generalization. Like when a teacher tell the "moyenne" (take all the notes of a class, add them together and divide them by the number of notes, sorry i'm french and word slip sometimes) so the teacher tells that the "moyenne" is bad for the class and they should do better, not all student have a bad note but the teacher won't name each bad student.