Thanks for the additional information. I'm sorry that I was not online earlier. I do think that you are entirely correct in your evaluation of your WHP female. She does sound very dominant.
I'm sure you are aware that dogs are pack animals. When a dog is a puppy (under a year of age or so) any older dog in the house is the alpha dog. When a new dog is brought into a household there may not be immediate fighting as the new dog may just think they are visiting initially, but once they realize that they are there to stay, the fighting may start. With younger dogs, the older dog establishes her/her rank over the other dogs with minimal or no fighting involved. Your other female likely is happy in her role of a lower ranking pack member and thus the WHP is ok with her. Now that the younger dog is matured, she feels she has to put the older dog in its place and establish herself as the alpha dog especially since she is larger, stronger, and younger. . An older dog will usually not just submit without some sort of altercation and this can be an ongoing issue.
It may also be that the younger dog senses some weakness (illness perhaps) in the older dog which is triggering the fights. Sometimes it will look like one dog started a fight, but with dogs a wrong look or movement by a submissive dog can trigger a fight. The mouthing behavior is often a more passive way of showing dominance.
Since she is stronger and younger, she is likely to be a good choice as the alpha dog. So you may need to adjust your interaction with the dogs since the alpha should be the one that is fed first, gets treat first and affection first. This will help the JRT see that you support the WHP's leader position. Since he used to be the alpha, he probably does subtle little things that the new alpha feels are disrespectful and that can trigger the "new alpha" to try and reprimand him with a growl or even nip. If he hasn't accepted her as alpha, he will fight back leading to a lot of altercations.
I would be sure to work with both dogs on obedience training. The following site is helpful for teaching you how to train your dog. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions. Use high value treats like hot dog slivers as treats if you want them to learn quickly.
Training works best if you train at least 30 minutes a day (two 15 minute sessions). I would start making your dog work via the Nothing in life is free program (NILF). It is outlined below.
Get them off the furniture. When allowed on the furniture, they tend to think they are the boss or alpha even over the humans or at least equal to them. You want to keep them on the floor so they see you as the ultimate boss. If you are the boss, then YOU say who gets to do what and not the dogs. Also only the boss is supposed to be reprimanding members of the group.
Also some additional training can help. Both dogs should be leashed and if one dog even looks at the other dog, a correction should be done. Any sign of aggression including a prolonged look, hair raised on the shoulders, a growl or even a stiff legged walk, should be corrected. A correction is a quick tug of the leash and a firm low toned "NO". Once you have done this couple of times, you should notice the dogs ignoring each other. When that happens, you will want to reward them for the desired behavior. Again, use tasty treats like the hot dog slices. This teaches the dogs that you WILL not tolerate fighting in YOUR pack.
The following sites go over dog body language and may come in handy to determine if the dogs are likely to get into an altercation.
I hope this information is helpful to you and you are satisfied with my response. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have .