Thank you all for your responses. I wound up making a second cake--all vanilla--for the allergic guests. BTW, I still have not received an answer from Durkee about what 'artificial flavors' they use. 
@Kachina624 your suggestion for amaretto is a good one I had not considered! For future reference, I will be sure to ask the guests about it. Besides for lab. chemicals, did you know artificial almond flavor comes from the kernels of peach pits? I've tasted them before, and they do look and taste exactly like almonds. That is what I suspect Durkee's flavor is made from, which would have been perfectly safe for my guests, but they're hesitant to expose themselves to liability. (?) I dunno.
BTW, if anyone is interested, I posted a photo of the cake that started all this brouhaha, a 3D chess board, in the Your Favorite Cake thread.
Strive for respect instead of attention. It lasts longer.