Prediction Page, (bring your quatloos, gps, and internets, you'll need them.)

[Peelee]

So, one-way memory, I'm forced to present a rough example: Greg set up an ambush for the party based on what he knew of the party via Durkon's memories. Then the party surprised Greg with completely different tactics, which were helped by the presence of Hilgya as a wildcard.

Imagine reversing the positions. The party plans an ambush for a character confident that they have fought the character before and know their weaknesses, then the character arrives and presents different weaknesses, facilitated by a wildcard, for dramatic effect.

Ignore Sabine for a moment in this example, consider it generic.

How does Rich set up this kind of surprise for the party if having his villains be consistent took top priority?

Alternate question: What priority is consistency and what is ahead of it? Incomplete answers are fine, you don't have to list everything and you don't have to rank everything exactly.

Ok, but none of this addresses "character is explicitly shown to ve vulnerable to this specific metal, as an immutable charictaristic due to what she is as a creature".

The bet is that she will not be vulnerable to the metal she has explicitly shown to be vulnerable to. You can pontificate all you want but that doesn't change facts presented in the comic.