This is the solution!Put a T in line with the return and a ball valve off the the T to return water back to the sump. That way you can control the amount of water going back into the tank vs. what goes right back into the sump. I can take a picture of mine if you don't see what I'm getting at, just let me know if you need it.
This is what I would do. Put a ball valve on the output of your pump rather than trying to control the water entering your sump.another option is simply install the valve inline and throttle the output to the needed flow. I use a little giant for my return throttled back about 22% and have monitored with an amp meter the current draw. It actually decreases right to the point of dead heading the pump then it spikes.
Restricting the output is normal mechanical practice it's restriction of the inlet that damages a pump. Working in a power generating plant 99% of our pumps have a throttled discharge and this is no different other than being smaller