I am sorry to hear you are worried about Jetta. My name is Rebecca. I am a veterinarian and will do my best to help.
It sounds like a lot of things have been tried. So she stopped coughing when on all the medications you listed, but now she is coughing again? Is she taking the hydrocodone/homatropine now?
What cardiac testing was done: EKG? Ultrasound? Consult with a cardiologist?
The first thing I would try to determine is whether this cough is "upper respiratory": from her nose, throat, and large airways (trachea, bronchi) or "lower respiratory" (lungs). If her chest Xrays appear clean then it is probably upper respiratory.
So far cough suppressants (torbulin, hydrocodone), antihistamines (Benadryl), treating for kennel cough (doxycycline), and heart/blood pressure medication (enalapril; and why that if her heart is OK?) have been tried. How long was she taking these? All at the same time?
My next step if this were my own dog, or a client's dog, would be to do a course of a different antibiotic along with the hydrocodone and possibly a "bronchodilator" like theophylline to see if the cough improves. If it does, this may be chronic bronchitis. I would wean off the antibiotics first but probably continue the other two, gradually reducing the dose, for awhile, probably several weeks.
Another thing to consider is a "trachea wash" to get cells and samples of fluid from inside the airways for analysis and culture.
Let me know if this is helpful, and what other questions you have.