In regards to the "Warrior of Darkness"; how far is too far?

[Iscah]

From the trailer, we can surmise a few things:

I think you're misinterpreting some things - or at least interpreting them in a more specific way than they could be intended.

1) Some sort of calamity has happened. Maybe not "the" calamity, but something really bad that causes lots of fire and destruction.

The "fire and destruction", unless you're talking about Urianger and Y'shtola's scene, seems to be the Ghimlyt Dark battlefield.

The bigger indicators of some kind of Light-aspected calamity are the strangely-lit wasteland where the WoL is, and the manifestation of Light-aspected monsters.

2) We know that the cause was that we tipped the balance too far to the light.

The likely cause of it all is tipping the balance, but we can't actually be sure at this point that it's "our fault".

From the scene with Solus and Varis, it seems that Solus is scheming something involving the Black Rose and, from memory, some kind of Light-aspected effect on it.

3) The warrior of light we see at the beginning seems to either have lost their memory, or are from another world, because they only learn about the jobs they've been and what they have done through the Echo.

I don't get that impression at all. He seems to be having flashbacks, Echo-visions of his own past, but that doesn't tell us he's lost his memory. It could just be a fancy story-telling device for the trailer showing us "how we got here".

If anything, I feel like the taunting gremlin would say different things if he was struggling to remember who he was.

4) Master Matoya has taken over Y'shtola's body. She talks about "our friend" having "gone somewhere" and that she will have to "hold the line" until they return. Now, she could be talking about Y'shtola (who's soul has gone missing), or she's talking about the warrior of light.

It's likely that Matoya is somehow in Y'shtola's body, but the details are still entirely unclear. Is Matoya in control? Is Y'shtola in there as well? Is the whole thing a deliberate misdirection?

Another possibility is that Y'shtola has returned, but at some point (probably after that) Matoya has willingly sacrificed her lifeforce to keep Y'shtola alive - and ended up in some kind of two-souls-in-one-body arrangement. We've seen it happen before in this story.

5) Urianger talks about "history needing to be rewritten".

You've got your quotes crossed. Urianger's line is about accepting or denying a foretold fate.

It's the mystery voiceover person who says history needs to be - not quite "rewritten", but "unwritten".