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I do stun by reducing the unconscious rating rather than adding it to damage, but I’d like clarity on this too.

That method works for Earthdawn, where it seems that the combined damage is compared to the rating, but the wording in 1879 seems to say that you don't fall unconscious until ether physical damage or stun damage hits the mark, not the combined damage.

There are two different types of damage: Physical Damage and Stun Damage. A character’s Death or Unconsciousness Rating represents the amount of each type of damage the character can take. When your character’s damage (Physical or Stun) equals or exceeds their Unconsciousness Rating, they fall unconscious. When their Physical Damage equals or exceeds their Death Rating, the character dies.

The word "each" in "each type of damage" and "or" in "Physical or Stun"
I mean it seems clear what it is saying, I am just not 100% sure I believe they meant to say that! It basically means that you can take twice as much damage (so long as it is split between the types. Also it means that strain does not actually affect you unless you take a whole lot of it, or somebody else is also doing stun damage to you. I mean this seems big!

That really should say "and/or". Stun and Physical add together to determine Uncon. Stun cannot exceed Death Rating and does not count toward Death Rating once Uncon has been hit.

This has been added to the Errata topic. Creating a proper Errata document is on my To Do list.

I normally try to avoid being pedantic, but when editing rules it helps if somebody is...
In the errata, I think that "and/or" is still not the correct phrasing. "Or" means that one or the other has to equal or exceed it. "And" means that both have to equal or exceed it. I think what you want here is "plus" as in (Physical plus Stun).
Also, the earlier phrase "each type of damage" also really needs to go.

So the edited text would be

There are two different types of damage: Physical Damage and Stun Damage. A character’s Death or Unconsciousness Rating represents the amount of [each type of] damage the character can take. When your character’s damage (Physical [or] plus Stun) equals or exceeds their Unconsciousness Rating, they fall unconscious. When their Physical Damage equals or exceeds their Death Rating, the character dies.

I think the above more unambiguously says what you meant it to say.

That having been said, have you seriously considered not errataing it? I was surprised as heck to see the words "each type of damage", and I felt It would make a huge difference in tactics that I was surprised I had not read any discussion about, But I was never certain it was wrong.

We have to errata this. Leaving Physical and Stun separate would be a significant break from the way CoreStep is meant to work. There's one Damage counter, and both types of Damage must apply to it. As far as editing goes, the Errata document will go to the rules mechanic and the line editor before it goes live.

This is related to my previous question, but is taking a slightly different angel.

Several different places in the Spellcasting chapter it says that Strain can kill a spellcasters.
I just want to tripple-check. Strain is stun damage. Spellcasting strain is stun damage. Stun damage can't kill a person. Correct? So is the statement that strain is stun damage incorrect or the statements that spellcasting strain can kill you incorrect?

On the other hand I assume that Warping is physical damage, and can and does kill spellcasters?

We're working this out. For now, assume that Strain cannot go past Uncon, and that the statement that Spellcasting can kill is incorrect. Warping damage can and does kill, yes, just like in Earthdawn.

Is it strictly necessary to use both the sun and warping damage? I’ve been using just strain.

Warping damage only occurs if there's a warping result from damaged astral space. Yes, you take the Strain for doing the spell, and then, if you're in a bad area, check for Warping. This was a concept present in Earthdawn. Since the chance of warping in an Open area is low, I normally only check for Warping if the caster is in a toxic zone, like a Crimean War battlefield graveyard. If you read through the NYC Actual Play blogs, you'll see that I'm not hitting Thomas for Warping damage, although technically all of lower Manhattan ought to count as Tainted. Once they get to, well, spoilers, he and the GMC spirit worker are both going to be dealing with a high probability of Warping damage.

Which book has the spell creation rules?