Welcome!
My daughter has a pistachio allergy as well. (She has a lot of other allergies, too, though.)
My husband has just a single tree nut allergy, though.
Just out of curiosity, when the ER administered epinephrine, did it seem to help almost like turning off a light switch? If so, then that is a pretty strong indication that this incident was definitely anaphylaxis and not a choking incident. Epinephrine won't shut off a choking incident, but it sure works well on anaphylaxis and airway narrowing from inflammation. If the epinephrine and other meds DID NOT seem to help... then that points to a possibility that this was a mechanical airway obstruction instead. I'm assuming that the physician wasn't able to see anything, or you'd know for sure?
Certainly follow through with testing at your allergy appointment (call a couple of weeks ahead and find out if you need to avoid any medications and for how long beforehand), but realize that it may not tell you much that you don't already know. A positive test result doesn't mean much without a reaction history-- and it sounds as though that history isn't as clear as you'd hope.
Do you have a prescription for an EpiPen? If not, GET ONE. TODAY. That device will need to go everywhere that your child goes.
The only nut that I would caution you about in addition to pistachios are cashews. They are quite closely related and seem to cross-react in a pretty high number of individuals. This probably means that mixed nuts should be put away until after you know more. But peanut butter should be fine, if she's been eating it regularly. Unless you have additional problems, I'd continue to do things more or less as you always have. I wouldn't go through your house and dispose of everything with a peanut warning just yet, in other words.
As you observed, a pistachio allergy-- in and of itself-- isn't that difficult a thing to manage. That's the good news!!
One helpful thing is that they are pretty much ONLY grown in one region of the US and in the eatern Mediterranean, which limits their ubiquity in both cuisine (restaurants) and in food processing.
Places where we have found that pistachios tend to show up:
shells can appear on beaches, in parks, people may munch them as "on-the-go snacks" (leaving the shells wherever they please.. in shopping carts, on store shelves, etc. GRRRR) and they are common in Middle Eastern cuisine. The shells very definitely can have enough residual protein to trigger a reaction, by the way.
Good luck!! I'm very glad that you recognized what an emergency this was and were close to a hospital. My daughter's first reaction, we were also very fortunate to live just a few blocks away.