Hey Alison,
Everything I've read, and all the advice I've been given says the cough medicines are no good at all- that is why there are so many....
Is the problem that she coughs because she can't get enough breath? or is too congested? (cough medicine might dry that out a bit for nights- but that dose might not be safe)
I have children w/ reactive lungs- meaning any time the saesons changed or s little cold bug was in the air they'd get a coal miner's cough and runny nose. When they were small like your baby, I's sit them up in the stroller, and strap them in for sleep. That let the mucus drain down instead of into the lungs. Because the air passage ways are so very small at this age, it made a huge difference. And while we didn't sleep through the night- we got some good 3-5 hour stretches in, and I felt less zoned the next day.
You can put a pillow under one end of her mattress and prop her up that way too.
Do you have a humidifier? One that blows warm or cold air? Adding mositure to the room can ease the cough, if it dry.
Bring her into the shower, or sit her by the shower while you are in--lots of steam and hot baths to help her drain. Putting eucaliptis oil in an diffuser while she is in the bath or sitting in a bathroom with the hot shower running also helps. You can do this in the middle of the night when the cough is bad.
SOmetimes milk products cause extra flem or mucus to build up. You can try avoiding dairy products for a few days and see if that makes a difference.
My kids usually had the cough for about 10 days before I tried these ideas.
Good luck,
M.