Car seats! There is no reason that this would be unsafe. Properly securing an appropriate car seat is vital to the baby's safety. Safe driving practices are obviously essential too!
Yes. I would recommend stopping about once an hour to give her a break and change her diaper if needed. Also, get a mirror to go in front of her car seat so you can see her in the rearview. Otherwise, as long as it's an approved car seat properly secured there is nothing unsafe about it.
It's soothing. Assuming asll is safe and the driver is alert and takes breaks, the motion may be soothing, even serving as a release from colic.