I agree with the others here-- particularly TT's analysis utilizing Occam's Razor.
There is no food without risk-- just some risks much lower than others.
If you know that her threshold is very low, then that means that everything is on the table (so to speak) as a cause. In that case, I also agree with Maeve that bread is a likely culprit. This is the reason why we've never been able to use store-bought bread (and for many years, pasta). I don't use preprepared broths for the reason you covered-- too many of them are run on shared lines for my comfort zone, frankly. I do use Better-than-Bouillion, even though there is some risk there for me personally given that they make a variety which is potentially SF-contaminated. My threshold is higher than DD's, so the risk-benefit for us personally works better there.
Since the symptoms were subjective rather than objective, too, it's possible that like YKW's DS2, this wasn't specifically IgE-driven, but was something else entirely; either thermal or texture, or-- well, something. Salt?
Anything on DD's face can still give her wicked hives from contact. It doesn't need to be especially acidic, nor does it need to contain an allergen.