Yes, please turn her back rear-facing. It is much, much safer. In answer to your question, there really is no "20 pound rule" since the "one and 20" line comes from the early 90's, when the first car seat laws came into effect, and since 20 pounds was the upper rear-facing weight limits on the (limited) seats available, that became the law. In essence, the law was "rear face to the limits of the seat" and later, when manufacturers began upping the weight limits to 30, 33, 35, 40, and now even 45 pounds, it's still recommended to rear-face to the limits of the seat.
Here is some information on why RF is safest:
This is the basics, but the link goes into much, much more detail:
(from the Child Passenger Safety Technical Encyclopedia)
|When the child is rear-facing, the head, neck, and thorax are restrained together by the back of the CR in a frontal crash. There is little or no relative motion between the head and torso that could load the neck. If the same child were facing forward, the harness would restrain the torso, but the head and neck would pull and rotate forward, leading to the potential for serious upper spinal injury
|Rear-facing car seats spread frontal crash forces over the whole area of a child's back, head and neck; they also prevent the head from snapping relative to the body in a frontal crash.
ETA: I was posting when you posted your most recent post. Screams or no screams, of course, she's safer rear-facing. I understand it's horrible to listen to, but there are lots of things you can try. What seat is she in? I assume she is in a convertible? Most babies hate being reclined to the 45* angle their infant seats require, and are much happier more upright. A convertible seat can be installed between 30 and 45 degrees from Horizontal, with older babies and toddlers preferably in the 30-35 degree range.