Update:
Elfie's up and around and has stopped the yawning movement. I killed the yellow jacket and removed it from the pen. Of course, there are a few around the yard at any given moment. Mostlly they leave us alone.
Anyway, I have relaxed just a little bit. Everyone is acting normal, no one is limping, or shaking her head with her mouth open, or any of that. I did find somewhere a recommendation to use ACV to take the sting out. So I poured a fresh pan of water and added a splash of ACV to it. Did Elf show an interest? Of course not!
Looking right now as if she did not have a strong reaction to the sting, if indeed my detective skills are worth two cents. All I know is a duck yelled, there was an injured yellow jacket, Elfie seemed to have something uncomfortable in her mouth for several minutes.
It would still be a help if anyone has seen dire results from mouth stings, and knows what to be prepared for. My guess is that like humans, some ducks may be allergic to stings and others just shrug them off.