Thanks for the detailed analysis, good to have some more realistic figures! Where are you getting your IntelliFlo energy use numbers from? I couldn't find any published power use curves for IntelliFlo.
It makes even more sense to go with a single pump if a less efficient pump is used. If all were single speed pumps, then the energy costs of a single pump is always about 1/2 that of a dual pump system. For energy efficiency reasons, it is never a good idea to go with dual pumps.
All pumps can have throttling valves on the return side of the pump. Throttling valves just add extra head loss to reduce the flow rate. However, for energy efficiency reasons, it is not always a good idea. You are basically reducing flow rates without reducing energy use very much.