After taking the seat out several times, adjusting the harness in several different height slots, and reinstalling it to see how both girls fit, I think I can see what the problem is. On the underside of the seat is the shallow groove/indentation for the harness path, leading up to the front of the seat to the recline bar mechanism and the harness adjuster slot/path. When the seat is installed correctly with a rock-solid installation (again, I am using the LATCH and top tether), the bottom of the Radian compresses fairly deeply into my vehicle's fabric upholstery. The thin harness webbing moves fairly easily underneath for tightening/loosening the harness, but once the SafeStop portion is pulled under the seat, it seems to have much more difficulty
moving along underneath the seat base. After it has stretched out a little bit, with the fabric covering of the SafeStop bunching at the back of the base and seat bight, I am no longer able to continue tightening it to the point that I need to for my particular sized child(ren). I can only assume that this is not the intended way that the SafeStop should function, with the cord stretched taut perpetually, and the fabric bunched up at the back of the seat.
I tried a neighbor's hefty 35 lb. 2-1/2 yr. old in the seat, in the 4th harness slots also, and I can get it tightened adequately for her,
barely-- I have to pull quite hard to get the SafeStop stretched up under the seat a little bit, but can't go further by more than a cm. I
assume this issue would not be much of a problem for larger toddlers and preschoolers.
Also, I changed the harness slots to the topmost position and reinstalled the seat, and with that configuration there is no issue with
tightening at all. But in any position below that, the amount of slack in the harness increases, causing the SafeStop to be pulled into
position underneath the seat base and stretched in a way I can only assume affects its intended function.