If its the same cross sectional area, then yes, a proportional decrease in volumetric flow rage (gph) will be the same as the velocity (ft/s). The thing is, our drainpipes dont just transport water, they also tend to suck in air since as the water falls, it picks up velocity and wants more volumetric flow but it cant get more water. Since no more water is provided, it sucks in air. So from the standpoint of air and water combines, then no, velocity is not proportional to flow rate, since less water can mean more air.
When you add the ball valve, put it on the very bottom of the drain line, right before it spills into the sump.
Here is a simple diagram of the method... You really dont even need the elbows on the standpipes either.
The valve on the primary (shorter standpipe) goes lower, I just couldnt show it on that diagram.