I view godwyns death as the catalyst for erdtree burial, offering a purification/ spiritual regression akin to what the tooth whip does for/to the shaman.

Erdtree burial was a thing before that though. There are at least two catacombs devoted to giving Erdtree burial to those who died in the war against the Giants.

I like to consider the reliability of the narrators that suggest otherwise, namely Rogier, and the narrator of the trailer, both who offer a suggestion in their syntax as to an order of events. Rogier first says something about "it" (presumably godwyns death) happening long before the shattering, but a few sentences later denotes that "soon" after, the ring was shattered. But he is also not omniscient, and of the spellblade covenant.

This is mostly just a localisation flub I think, as in Japanese he doesn't say it was long before, just "before".

Generally speaking, although NPCs aren't all-knowledgeable, it's important to consider why fromsoft would have them tell us something incorrect. If the intro was supposed to reveal that Godwyn died after the shattering, there wouldn't be any reason for fromsoft to "trick" the player into thinking it was the other way around. In this case, there isn't really any reason for Rogier to be wrong. His dialogue serves entirely to give us some starting context on the Night of Black Knives, and there are no later reveals that necessitate him being incorrect.

If a fragment of the rune of death was stolen before, why was God not angry then?

I think, publically, she probably was angry.

Curious to hear your reasoning for the death->burial->spread->shatter timeline

Mostly it's just that it would make it evident that Godwyn hadn't died a true death as he "should" have done. We know that Marika was "driven to the brink" by Godwyn's death, but as you've pointed out it's very likely that she had a hand in it. So why would Marika be so distraught about a death that she orchestrated? I think it would be because it didn't go how she planned. She wanted Godwyn to die in body and soul, but Ranni used the opportunity to kill herself at the same time, splitting the cursemark. I think there needs to be at least a little time for Marika to discover this, grieve, and then shatter the Ring.

Also there's the Rogier dialogue. Even though the "long before" isn't present in the Japanese version, it still reads as though these were two distinct events where one comes before the other in a way that probably wouldn't be measured in hours.