Not an expert, but....

The valve at the skimmer does the same thing as the valve at pump, so you could pick one and eliminate the other (I moved my skimmer valve to the pump because it was easier to get to). Can't see why you would need one after the filter. Having one at the return has been VERY handy for me for occasional maintenence on a solar heater.

I also have one between pump and filter to regulate flow.

I will say ball valves that come apart act great as quick-connects for easy tear down and maintenance. Hope that helps!