I wouldn't leave your LO in a carseat for more than two hours at a time. Although she prefers it, carseats have been proven to create severe spinal problems later in life, such as scoliosis and spina curvatures.
I agree about rolling up blankets beside LO, or maybe buy a sleep positioner that will have padding down her sides. It could be the angle she prefers, so maybe a bouncer would do it.