There's only one part of your post to which I might have some useful input. I have many of the same questions...
Also would like to get a simple design for solar heating for pool water if anybody has one or a link to one.
A few years ago, I built a 4-foot by 8-foot by 6" box with a clear plastic cover.
Inside this box I put a 4x8 sheet of galvanized metal, painted flat black. On top of this sheet, I put a bunch of 1/2" copper pipe, running from one end of the box to two 90 degree elbows, then another pipe going the opposite direction, until I reached the bottom of the box. I ran the bottom (input) and top (output) parts of this tubing grid through the sides of the box, then painted all the tubing flat black.
I set the box up so the clear side was facing the sun, then took a pond pump and connected it to the input and dropped the pump into the pool. Then I ran an outlet hose back into the pool.
I took copious notes during this project (until the rain took its toll on the box, which was just a test-of-concept prototype anyway) but doubt I could lay my hands on them right now. But I'm thinking the water at the outlet was 40 degrees warmer than at the input.
Commercially produced solar pool heaters are pretty easy to find and work like the setup I built. I felt I could improve on their efficiency, though. Especially because the ones you can buy aren't sealed up in a box. My version let me capture some of the very high ambient temperature inside the box.
I later built a copper tubing heat exchanger to go in an old wood stove and routed the output of the solar collector to the input for the wood stove, then out to the pool.
The heat exchanger in the stove kept coming apart because sometimes the water flow would get interrupted and the solder on the fittings would melt.
But when it was running, the output was extremely hot. Scalding if touched directly. I ran the stove outlet into the skimmer so it could be mixed with the colder water going into the big pump before coming out the main pool outlet. Was fun standing in front of the outlet in the pool because it was so much warmer than the surrounding water.
I did this experiment at the beginning of May and we were swimming in the pool before the end of the month. Usually the pool isn't warm enough until July.
I was able to raise the temperature of 20k gallons of water 2 degrees per 8 hours. I'm sure there's a formula for calculating the BTU's that represents, but I don't know the formula. I just know it was a bunch.
The downside, and why I haven't made another heater, is that instead of the pool being warm enough from July to September, it was too hot. Upper 90's and no way to cool it down. Not at all pleasant for swimming.
Someday we do plan to enclose the pool, though. And once we do, then I can fiddle with neat, green, ways to not only keep the pool warm in the winter, but harvest heat out of it for the workshop.
By my reckoning at the time, if I had a blanket to keep over the pool at night, I could've taken the water from 60 to a swimmable 80 degrees in barely a week. With the only energy used coming from my plentiful trees. Especially Osage Orange which I think has the highest BTU output of any wood available and is plentiful around here.
With only the solar heater, it might've taken a month. And if I didn't get the pool into the mid 80's in May, it likely wouldn't have been too hot for swimming in July.
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