I don't know.
has contamination from the mail been ruled out for her case?
It's hard to prove a negative, but there's no specific evidence that I'm aware of that it was due to cross-contamination. It's probably tentatively characterized as cross-contamination in the absence of any other known delivery method for the spores that infected her, but that's hardly proof or even evidence.
All I know is that this is the one anomalous case. If I were at the FBI, that's where I would start looking. But I'm not, so it's moot.
"how do you think the Kathy Nguyen death fits into this?"
For me it was very suspect she worked in a medical/biological facility. But then, many people do.
Don't forget the old woman in Connecticut who got it. To me that's more anomalous, geograhically. Some mail cross-contamination theory seems more plausible for Nguyen, being she worked in Manhattan where some of the letters were sent.