Sounds like some Rube Goldberg engineering that you're planning. I have a drainback solar hot water system in operation for a couple of years, and I've installed many systems, including pressurized glycol & drainbacks. I think you are needlessly fretting over the additional energy consumption for the high head pump requirement of a drainback system. What you can do to minimize the pump size is to locate the drainback reservoir as high up in the thermal envelope of the house (insulated attic if you have, or just second floor ceiling if not) - this will reduce your pumping head height significantly. Then, you can install the smallest pump based on your head & flow requirement down in the basement near the storage tank or heat exchanger. Also, you should jsut keep the circulating pump between storage & heat exchanger on the same circuit as the collector pump - don't fret about the lost btus that fall back to drainback tank upon pump shut down. If you want to get obsessive about pump energy, then start by using a Wilo ECM pump in series with a higher head pump, and use a 5 minute delay off timer for the larger pump. So, on start up, the differential controller activates all 3 pumps, 5 minutes later, the high head pump shuts off, letting the free fall of water returning down the pipe work for you, requiring only the power of a small circulator. As for having the panel filled just once during the day, again, you're fretting over nothing. The solar panel can take the thermal shock of start-up & drain-down. Your differential controller should be set for 4 - 6 degrees F. shut -off and 16 degrees F differential start-up. Let it do its job.
Best of luck