So your adult Newfie is bossy with your 2 year Newfie and now is not just bossy but vicious to your Cav Charlie, can I ask the age of Charlie?
If Charlie is an adult I doubt it is due to the bitch being aggitated by your Cav's in the face behaviour as older Cav's IME are usually laid back, not jumping and irratating to annoy the Newfie.
If health is ruled out, I would say her behaviour at the 6 month mark is due to her now feeling completely settled and at home and is showing her true character. She does not appear to have any patience with other dogs, if neither dog is being bouncy or irritating towards her they are probably in quite a precarious position.
Do you know anything of her background, was she from a puppyfarm, shut away and not used to other dogs? Was she attacked by other dogs? Does she have any kind of history you know of?
I feel very sorry for Charlie, poor dog having to live upstairs for fear of your bitch, I wouldn't have that, the aggressor would be in isolation not the victim, she would be segregated anytime she showed unfavourable behaviour.
Firstly, get that medical check, secondly segregate the aggressor not the victim, thirdly get in a behaviourist, but most importantly get to the bottom of her history. See how things go, but personally my dogs of good character would always come before an aggressor especially a new arrival, I know you have done a good job in bringing her back to health but perhaps she is just not meant to share a home with other dogs.