Pony with heaves/COPD/Respiratory

We adopted 2 Shetland ponies (12 and 13 yrs old) in the spring. A bonded full brother and sister.

We did not realize she had respiratory problems until a few days after we had her home- the previous owner must have been doping her up with meds and when they wore off we realized she had respiratory issues.

We started her on free choice non gmo minerals, DE diamataceous earth, clay, kelp, liquid vitamin, loose real salt, humic granules, and she eats hay and some grass but not a lot as we don't want to risk laminitis - she gets NO grain. I have had her adjusted by my chiropractor sister in law but I can not find an equine acupuncturist or homeopath near me with enough experience in this kind of condition. With it being chronic I don't want to wade into this alone.

I have been managing it with high doses of vitamin c, msm and the other items listed above quite successfully however all this is quite expensive and we've been doing it 3x a day for months. She has certainly improved physically and mentally - her coat and hooves are fabulous, Due to these supplements her parasite count is almost non existent (despite 2 of her field mates being exceptionally high) As soon as I reduce the frequency of the doses of C & msm, the coughing resumes.

lately she coughs seemingly all night long, but hardly at all in the day. I can hear her out my bedroom window at night - reminds me of whooping cough without the whoop. On humid days, she rattles when she breathes too. I have given her Buckley's Horse Cough Medicine out of desperation some nights to get her SOME relief. Odd she has no heave lines though which you would expect if it had been ongoing for a long time.

She is a lovely little girl, a bit nervous and head shy- bottom of the herd, her brother who she has lived with her whole life bosses her around. I have very little history on her but she did drive with a cart/wagon as they came with their harnesses. We have asked very little of her since she arrived to give her time to heal - once the cough was under control my 6 yr old has been riding her this summer around the property...- but now that the cough has returned I have asked him to ride her brother instead.

She is NOT kept in a stable. No vaccines or meds since she arrived. She is out 24/7. They live on secluded property although the highway is on the other side of the trees. In the winter we have an outdoor woodburner stove - smoke definitely aggravates and makes the cough worse as does grain or mold. She is kept on a track type system (Paradise Paddock) set up that goes through the bush, around the outside of fields etc. to encourage them to walk way more and naturally rather than standing in a field)

Any help would be greatly appreciated. I had hopes that I could actually "cure" this rather than manage it for the rest of her life. I would like to do all I can for her. It is becoming harder to find information on the internet to research this myself. As well, coughs in my children were always difficult for me to treat assess and treat accurately with homeopathic s :-(

Thank you in advance to anyone that might be able to help us.