A couple of things spring to mind for me.

The first is the heat of the seat. My daughter hated being in her car seat if I had the shoulder pads and buckle pad in. She just got too hot and was so sweaty when we arrived at our destination.

The other thing is the padding under the babies head. Every bump and bounce along the road is felt through the baby seat which their tiny heads rest on. Are you able to put some more padding between the car seat cover and the hard shell. I just got some memory foam from Clark rubber and cut it to fit around the seat belt.

A dummy can help with colic, and it also helps with 'popping' their ears if you have to go up or down any mountains.

Last but not least, have you tried giving her a bottle in the carseat? My daughter is breastfed, but I expressed a small bottle for her and gave this to her as we started our journey. It was enough distraction to get going and soon had her feeling sleepy as well.

All the best, sounds like you have a tough road ahead.