I don't clearly understand some of your questions,therefore won't respond to those. My advice is (A) go to "Build it Solar" site for some tips (B) Unless the system include's means of boosting temperature as required, use a conventional gas or electric tank heater as storage for solar heated water. To the question about pump needlessly running, A drain-back system has built-in freeze and overheating protection. Because water only enters the panels after they’ve heated up, and then drains back when the panels cool,pump will not operate unless panel heats and freezing is impossible when panel doesn't heat. As for cloudy days,the panel will produce a signifigant amount of heat which will save energy required to boost tempature.

The thing is I gather that, in a drainback system, when the thermal panel reaches a specific (hot,) temperature, the pump turns off, and the water drains from the panel, thus preventing the possibility of overheating. However, if the weather becomes really cloudy for the rest of the day, does the pump stay on "trying" to heat the water?

Or how exactly does it work? I understand how the drainback works to prevent excessive heating and freezing.

I think I found the answer to the question about the pump staying on in cloudy weather:

When the water temperature in the storage tank gets hot enough, OR when the water in the thermal solar panel is no longer warmer than the water in the storage tank, the pump controller (with suitable sensors) switches off the pump, and the water drains back into the drainback tank.

So I assume that in very cloudy weather, the water in the solar panel collector would become cooler than the water in the storage tank and the pump would turn off automatically, provided a suitable pump controller is installed.

I found the information here:

https://sorel.de/en/controller/tdc-solar-controls/drainset/

Thanks