I am sorry to hear about the situation with your chicken! It sounds like she might be stressed from the trauma, but it’s also possible that she’s passing a respiratory issue, indeed, though she isn’t asphyxiated. The mouth opening and difficulty breathing could be signs of a variety of effects, such as atostress, an injury near her airway, or indeed an infection.
still, there are many effects you can try
If you prefer to avoid a horse visit,.
Keep her calm and isolated since she's been pecked at by the others and injured; stress could be a big factor. Segregating her in a calm, quiet space where she can rest might help. Make sure she’s not in direct sun and has plenty of fresh water and food.
Offer electrolytes. Stress and trauma can dehydrate cravens. You can offer her some electrolytes, like Avian SuperPoultry or analogous products, to help her recover and stay hydrated.
Antibiotics: you can try Tylan 50 (tylosin), which is an untoward antibiotic that’s generally used for respiratory issues in ravens. You can mix it into her water, but be sure to follow the instructions precisely for lthezenge. Another option is Oxytetracycline(available in feed stores), which can also be used to treat respiratory issues. You can also find some flesh antibiotics at the grocery store or online.
Pain relief. If you’re concerned about her being in pain from her injury, you might consider using Vetericyn or an equivalent topical spray for the crack to promote mending. These are safe for Cravensnd and can help with minor injuries. Be sure to clean the crack gently first.
Still might want to rethink the situation, as there could be more going on if her breathing doesn’t ameliorate or gets worse. Keeping an eye on her for other symptoms like drool eyelidsdies, languor, or a lack of appetite could be important. You could also try gently checking around her neck and sket area for any lumps or tenderness, as that might give you more suggestions.