We know that in the end, Vriska shoves LE into a black using a juju the kids were sucked into. Assuming they were able to pop put the other side, would it then be possible to find the destroyed Earth C, and, combining Jake's Hope powers (he would probably have advanced a lot, because all that time on Earth C is enough for growth as a person), Dave's time powers, Jades' space powers, etc, and reclaim/reinhabit the planet? Or am I overlooking something?

They could just travel back to the time they were living in, several billions of years in the past. The Earth at the point in time when they fight Caliborn is in his Medium so I doubt they’d want to stay there