Hi there,
My daughter is also 6 mo. old (today actually :). I can definately relate to what you are going through. Sounds exactly the same as my daughter! I relate most of it to teething. Lillie too had a bad cough and just seemed to have a cold for quite awhile... at least 2 weeks. She would cough at night so badly, we were scared she was choking. And her poor runny nose, sounded like she could hardly breathe. After about a week of the coughing, we took her to the doctor and just like you said, they tell you it will run its course and there will be many down the road.
When I asked my girlfriends with kids, they agreed that it was due to teething. The cough can be due to excess saliva from teething, draining down her throat, causing the cough. I also read that teething can weaken the imune system and they are suseptible to colds. Lillie has her two bottom front teeth in, has for about a month and a half. I keep waiting for others to come in because she sure seems to be in discomfort. She had also been sleeping through the night til all this. Started with one time getting up, to up to 5 times a night. The only thing sometimes to comfort her is a bottle, and she shouldn't be hungry, but I want to make her happy..... such a dilema! :) She is getting better and back to once a night but still gets the bottle. I actually just started not adding as much formula.
That's just my thoughts and when I saw an email that read like I could have wrote it, I had to respond.
K. - this is my first child, very new to things! :)