Re: Radian FF seated height?
Diono allows it because the height of the shell is low relative to the harness slots. If the tips of a child's ears were even with the shell, their shoulder would be over the harness by only about an inch. Diono has done testing with shoulders an inch over the harness and they feel it's an acceptable load on the child based on the width of the harness slots, the stretch of the harness webbing, and whatever other factors affect shoulder load.
You are correct that they are absolutely the exception to the rule. Diono also recommends LATCH to 80 pounds and completely disregarding the weight limit set by the vehicle manufacturer because they feel they can back up their crash test data.
Other car seat manufacturers say a harness must be above a child's shoulders for FFing so the harness isn't pulling down on the shoulders when the child is moving forward in a wreck.
IHA: 1 wool interlock soaker (M/L), 8 lacey bum pocket diapers, 1 Wahmies pocket diapers brand new (OS, pink)