Honestly - it seems you are projecting a lot onto the paint horse that might better be placed on you and your horse(s). YOUR horse has become buddy sour/herd bound, and you are blaming the other horse for that? Have you considered that your own feelings towards the paint are being read by your horse(s) and that is why their attitude towards you has shifted?
Actually, it's not ridiculous to believe the dynamic changed when the paint was introduced. The hierarchy in the herd can, and does, affect how the horse responds to humans when we're working with them.
Horses naturally seek leadership. When a horse (or human) presents themselves as a strong leader, the horse will naturally follow. So when this little paint stepped in and started portraying strong leadership in the herd dynamics, the horses just followed their instincts and felt comfortable with this pony telling them what to do and where to go. It's like a safety blanket for them. When you take them away from that dominant figure, you take away their safety (in their eyes) so they get anxious, panicky, and seek leadership. That's when it's your place to step in and reestablish yourself as leader.
There's a reason that most horses who are kept stalled and (mostly) secluded from other horses act differently than horses who are kept in a pasture with several other horses. When a horse is kept stalled, it depends on its human for leadership, socialization, feeding, exercise... When they're kept at pasture with a herd, they have a whole separate totem pole to explore, if that makes sense. They're taking care of themselves, for the most part, and they will gravitate toward whomever steps in as leader within their herd. You have to work a bit harder to establish yourself as the leader.
Try not to get frustrated with the horses - they are just being horses. Sometimes they make us work hard. You can probably learn a lot from this mare if you look at it from the right angle.