I feel for you! So stressful! How old is Mustard? I had a 1 year old Salmon Faverolles (Tezza) do that when I introduced 2 pullets. The other 2 adults were only mildly interested in the littles, but Tezza just flew at them, I was afraid she'd kill them!
I ended up temporarily rehoming her to a flock down the road. I think it was probably 6 to 9 months later I brought her back and now Tezza is the submissive hen. She is now 3 years old and the littles are now 2, and the older flock members have died off and now I'm faced with introducing littles again. I'm hoping you'll tell me Mustard is fairly young because I have a theory that younger hens are more volatile than older ones. Sorry to blab on and on, but I do hope you figure something out!
I think from what you said though that it's either see and no touch until the others grow enough to stand up to Mustard or Mustard loses interest in war, whichever comes first. That or rehome the troublemaker at least temporarily, it worked for me. Good luck! I'm anxious to see how this plays out!