If her nose, is runny and she has post-nasal drip... then as it drips, down the throat, THIS causes coughing. It irritates and/or tickles the throat. And the reaction, the involuntary reaction, in the body, is to then, cough.
A runny nose, causes, post-nasal drip.
It is nose, drainage.
It goes down the back of the throat.
So, even if she can breathe and/or is not clogged up in her nose, the fact that she has post-nasal drip, she will cough.
So, yes, saline nose drops and humidifiers may help. But, if the post-nasal drip is still happening, then it will cause coughing. As it drips down the back of the throat.
I don't know, per 15 month old babies, what else can be used. Surely, antihistamines, are for much older toddlers or older children. ie: things like Clartin etc.
It is good, the Doctor pinpointed her cough as not being in her chest.
As a kid, I had just general 'allergies' to dust/pollen/dander, anything in the air too. And on windy days. I always had a runny nose (clear) and my eyes would get watery (still does). But, I did not have to take anything for it. But that was me. As a kid. Now as an adult, I take Claritin, IF and when need be. Not just everyday. Only when need be. At certain seasons, it acts up more, because more pollen/dust in the air. I just react to general dust in the air.
As a kid, it really didn't bother me.
You may REALLY consider getting a good air filter/purification unit for your home or just for her room. THAT is my recommendation.
That will help a lot.
My parents got me one when I was a kid.
The humidifier/saline nose drops, will help IF her nose is CLOGGED. That is what it is for.
It will not STOP... the post nasal drip.
all the best,
Susan