A 4" pump inside a 4" shroud will have a little less than a 1/4" gap all the way around. I have tested this and know it will not restrict the pump at all unless it is pumping more than 60 GPM.
The reason you are having a hard time understanding the amps drop when the pumps flow is restricted, is because it is counter intuitive. You would think restricting the flow from a pump would make it work harder, and it would if it were a piston type pump. But a centrifugal type pump works more like a blow dryer or a vacuum cleaner. When you put you hand over the suction of a vacuum cleaner or a blow dryer, you can hear the motor speed up. If you have an amp meter you can also see the amps go down. That is because the blower or pump isn't moving any air or water, and there is no load for the pump. The more GPM a pump is putting out, the more weight it is lifting. 1 GPM weighs 8.33# per minute. 10 GPM weighs 83.3# per minute, and that is what draws amps, not the pressure.
Then when you look at the cooling chart that says a minimum of 1.2 GPM, that is at full load amps. When the amps drop just 10% or 20% the motor is de-rated so much it could safely pump hot water, so it only takes about 2/10s of a GPM of cool water to keep the pump/motor happy. The 1 GPM minimum built into the CSV is actually 5 times more flow than needed to cool the pump/motor.
If the CSV was an easy thing to wrap your head around I would have thousand companies competing with me. But since it has a complicated explanation for a simple valve, there are very few who can explain it. The CSV is also not for the average customer. A person has to educate him or herself the way you are doing to understand all the benefits of the CSV. People who don't want to know anything about their water system are doomed to repeat the problems associated with the old pressure tank only system or even VFD systems.