What are you using for the end of the sprayer, i.e.
, what is the hole pattern? Also, what are you using for a cleaner? Is it wetting the entire surface on the inside including the shoulder? If so, it might only be a case of running it longer. The stuff that dries up near the neck eventually comes off in a PBW-like soak. There are various garden hose sprayer ends you can use, but I don't know of any that would get you more than a upper hemispherical pattern. If you wanted to brute-force it, you could make the wand out of 1/2" copper, put a cap on the end, and take a small drill and drill a bazillion holes at different angles along its length, sort of the Zapap approach.
You could also go with water fountain nozzles: http://www.atlanticfountains.com/spray_patterns.htm
Whether you need a ball valve will depend upon the flow restriction coming out of the end of the sprayer and how much flow the pump provides. You'll also want to make sure your pump would be happy if you restrict its output like you can with the magnetic drive March pumps. I've never played with a sump pump, so I am not very familiar with how they operate.