1. Unknown.

2. The Warriors of Darkness were former Warriors of Light from a different world. We have no information on Warriors of Light from previous eras.

3. Yes; they are from the First Reflection.

4. It's implied the party in the trailers is on the Source, our world; the ones we encountered in-game are alternate universe versions of them.

5. It's not Hydaelyn's fault; excising all Darkness naturally leads to a Flood of Light. She sent Minfilia there to fix it.

6. See 5 above.

7. Urianger's gambit was to force Hydaelyn to free Minfilia by using our and the Warriors of Darkness' Crystals of Light, and Minfilia would then be sent to the First Reflection to deal with the imbalance of Light. This was achieved by having us clash with them until they attempted to flee to the interstice between worlds using their Crystals; we effectively used our own to piggyback on their trip and call up Hydaelyn at the same time.

8. Unknown. It's either that She's too weak to do anything about it, is incapable of intervening in the physical world, or (like with us post-2.0) She could not hear them.

9. They returned to the First Reflection, but as a consequence of the method they used to get here (i.e. Ascian-style planeswalking, which requires physical death) were absorbed back into the lifestream. In other words, they're dead.

10. It was a decision made in the heat of the moment. Dragon eyes are known to be bad things, and hanging onto Nidhogg's any longer than we really needed to would be inadvisable after Estinien ended up getting possessed. Nobody expected them to come back after being thrown into the gaping chasm the Steps of Faith bridges - so soon, least of all.