[FirstYorkie]

I hope the pro will be able to help you! Don't give up hope yet as this may well pass (especially with professional intervention).

It seems to be me that a luxating patella (a knee that slips in and out) would be extremely painful and would add to the likelihood of fighting. Goodness knows, I get "witchy" when I'm in pain! Did the vet recommend any treatment for that?

I don't think it's imperative that you identify who is the culprit. It takes 2 to fight after all. You are right that staring is an act of aggression in doggie language and, if you can intervene at that point, you might can stop the escalation.

Something that might be useful while waiting to see the pro would be to order one of Turid Rurgaas' books about Calming Signals. This would help you to identify when one of the pups is trying to say "hey, I'm not looking for a fight". You would then heavily reward that behavior.

Good luck!