Other things to try and explore:

• Replacing smooth copper pipe with crinkly tube (to increase turbulence in the heat exchanger)
• Try tar paper instead of roof flashing for the mylar backing
• Completely sealed tubes (using acetone? check Fossil Freedom) for drainback design
• Dual pump setup w/ solenoids for switching (one strong pump to prime, one efficient pump for solar panel-based running), for drainback initialization