OK, First, the only way Squall and Co could survive and travel in time, in time compression was because...
Laguna: As friends, believe in one another.
As friends, love one another, believe in there existance and they'll believe in yours.
It's all about Love, Courage and Friendship.

By doing so, it was the only thing they could do to survive in time compression, and sin is the embodiment of none of that.


HAHAHA
You really wanted to debate the meteor...
Lucrecia/Shelke: Omega is the same type of lifeform as the weapons we encoutered 3 years ago. Normally, Omega poses no threat, it only manifest itself when the planet detects something that may cause a danger.

The reason why the planet has weapons is to defend itself.
Without Aeris' Holy Materia that absorbed the Meteor, the planet would have been destroyed.
Or, if you so insist, without the holy materia of Aeris, the planet would have been drained of it's life force and died anyway.
Without Aeris' Holy Materia the planet would have been destroyed.