It sounds like Bailey's reactions are a bit stronger than Molly's. I've never had to give her benadryl for hives (phew!) and she usually just gets yeasty ears and some general itchiness.
Do you keep a food diary for Bailey? If it's hard to keep an eye on things or be consistent about taking notes, you can also just portion out her treats for a week and hide all the other stuff. I had to limit which treat was available for BF to give to Molly when I was doing process-of-elimination in trying to narrow down Molly's allergy trigger(s). It was hard to get him on board with making notes so it was easier to limit his access to stuff he was giving Molly,
I think there can be some value in trying to narrow down what Bailey's trigger(s) might be. If I had kept to a strict no-beef rule, I would not have been able to try a joint supplement for Molly that is derived from bovine yeast. Since treating her joint issues is kind of top priority, being able to try different things is important to me so given what I knew of her history, it was a safe risk to put her on that supplement even though it contained bovine-derived ingredients. Adequan is a popular joint supplement that contains ingredients extracted from bovine trachea - not choosing to try that one yet on Molly but I think her risk of allergic reaction to that will be low - it will be more likely that I decide not to give her Adequan for entirely different reasons.