The safety seat doesn't move side to side more than 1 inch nor can it be pulled away from the back of the carseat at the top at all. However, I can move the safety seat away from the back of the car seat at the bottom more than 1 inch. Is this ok? I only see information about side to side movement or being pulled away from the car seat at the top. I can't find any info about the safety seat being able to be pulled away from the BOTTOM of the car seat. Does this even make sense?
I should also add that this is a shoulder/lap belt install on a Radian XT. This seat is behind the passenger seat in a VERY tight 3 across install in a 2004 Toyota Corrolla. In the middle in an Evenflow Big Kid booster and next to that is a LATCH installed rfing Britax Roundabout. Honestly I think part of the issue is that the Radian is not actually sitting where it should on the seat. There isn't enough room so it is pushed slightly closer to the door than it really should be.
You should be testing for tightness AT the beltpath. Grab the seat where the belt goes through and see if it moves side to side or front to back in that area. If it's less than 1" you are good, if its more, then its not in there right.
Have you tried switching the seats around?