I am sorry to hear about Katy's condition. I am a veterinarian and will do my best to help.
This does sound like it could be congestive heart disease
. There are lots of good drugs for this, but they are prescription medications.
She could also have cancer in her lungs, or bronchitis. It sounds like she could also have arthritis
from your description of her hobbling around. It must be sad to see a beloved dog get so sick and old.
You could try a little aspirin. The dose is one low dose aspirin per 15 to 20 pounds body weight once to twice daily. It has to be given with food, so if she is not eating
well I would not recommend it. It could help with her arthritis and might help prevent stroke/blood clots from heart disease. If she vomits, do not repeat giving aspirin.
You can try Robitussin DM
for coughing, 1/4th to 1/2 the dose on the label for people.
Let me know what other questions you have.