I hate to be the bearer of bad news....sometimes you just get motion sick. And there is nothing to be done about it.
I was like your daughter. Every time we got in the car. It eased up as I got older and taller and could see out the windows, but the fact is, I still, to this day cannot sit in the back seat of a car. I also cannot ride on boats, planes can get dicey and trains are okay as long as I have my wrist bands on and have taken a dramamine.
Chiropractic, acupuncture, no inner ear issues, - nothing has helped. I just get motion sick.
So, that being said - the things that help are mostly what you have listed. Keeping cool, being able to see out the windows in front, NO reading or looking down at activities, making sure my stomach isn to empty - happens faster when I am hungry, having wind on my face (seems to help my body and my brain get together on the idea that I am moving), wrist bands help for mild movement, and dramamine/bonine. Being able to sit in the front seat made the largest difference.
I learned early on to put myself to sleep in the car when we had to go someplace. I still do that if I am not driving. Mostly I drive, because I don't get sick when I am piloting the car.
It sucks. I'm so sorry for your and your daughter. I wish I had better advice. :(
I have a friend who has her daughter listen to books on cd in the car. It works for them after trying everything else.
Why is she being treated by doctors? Why a chiro?
Perhaps her motion sickness is involved with whatever heath issues she has.
As Adina said, sometimes motion sickness just can't be helped. But I have sat in the back of the car with Juniper and helped her gaze at far off objects, for looking at the furthest possible distance has helped reduce this vomiting. Looking at close objects while travelling makes one go slightly dizzy as one's eyes cannot keep focussed. Another tip I found helpful was making a tisane from mashed ginger root steeped in hot water and allowed to cool. Ginger as you probably know has good anti inflammatory properties, so a little sweetened ginger drink of this tisane won't hurt. Persist with the ginger chews; even long term they are helpful. Even during a time of rough seas on a cruise our family went on last week, chewing crystallised ginger worked out very well.