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

[Peelee]

She could be homebrewed! I'm holding on to my gold until we fully confirm!

In any event, as already linked, adamantine was never an option.

Not everything is about the character. The thinking was clearly plot-centered: Haley ran out of adamantine arrows, that could be significant in the future, let's pick a character it could be significant for and bet on it.

Rich has to show he's not going to use spells, magic items, or IFCC shenanigans to change out Sabine's weakness for the sake of the plot, because "Oh no, cold iron doesn't work!" is the plot point bet on.

Yes, but again, she has already been established to not be particularly susceptible to adamantine. This goes to what the author has said on consistency. He doesn't care about keeping to the exact rules of D&D anymore, but he does care about consistency, and her damage reduction is overcome by either cold iron or silver, depending on if she was a demon or devil. One of these is confirmed to work (linked above), she is confirmed to be a succubus (linked above), and thus we can absolutely conclude that the cold iron is what she is susceptible to. Changing this would be inconsistent, which the author has explicitly said is what he considers more important.