Erm...
While it is a bit hypocritical of Eorzeans to use Omega after chastising the Garleans for using Allagan tech at any and every opportunity without considering the consequences, the difference is that we're like to use it only as an absolute last resort and put it away once we're done. The Garleans would use it at every opportunity to ease their path of conquest. Eorzea? Smoking crater! Beast tribes? Consider them gone! Those Ananta we're going to meet would be little more than an afterthought if the Garleans got their hands on Omega. That is the difference.
As to what the explosion at the end of the trailer is... either "Super Griffin" breaking out of "White Dalamud," Omega's Wave Cannon, or Omega's Wave Cannon blasting White Dalamud to hopefully kill "Super Griffin." It can't really be much else.
When I said the game is biased toward the Ascians' victory, what I meant was all they have to do is... do things. Action creates change, change creates conflict, conflict creates chaos. We can't win.
There is like to be moral ambiguity as a theme in Stormblood; we Americans have romantic notions of revolution owing to our country being born from one, but they are often messy affairs without a plan afterward. There are also extreme actions in most revolutions, as displayed by the Griffin and his Masks - this is like to continue. On the other hand the Garleans are almost certainly tyrants, as shown by their actions in Doma and their general dealings with us. Neither is a good choice... we need a middle road, but nobody will listen.