Colin...need more pool specs to give you a good answer... you have a 12' round AG or a 20x40 inground? How much rise do you have to the roof?
Billybob...In a properly sized and functioning solar heater, when running the, pipes should feel almost "cool". Your hand would be about 96 degrees - probably too warm for your swimming pleasure. You can check my math but 1/2"ID pipe x 75 feet (pi X r-squared X 75 feet) you're only heating 14.72 cubic inches of water...one cubic FOOT (1728 cubic inches) is only 7 1/2 gallons. I've got 300' of 1" pipe ($19.00 ea. at Lowes) coiled on the ground (takes up about a 9-foot circle hidden behind patio bench) and with a 330gpm clear water pump rated for continuous use, 1/2 hp, 10' lift, ($25.00 at Harbour Freight), when the sun's out I get about 12-16 degree rise at the pipe outlet (the pump has 1" connections so I have a 1 1/2" adapter on the inlet). You're right about the nighttime cooling effect although it wouldn't cool it beyond ambient temperature. You should remember that your soil around the pool is an insulator so after it warms up you'll be more efficient. If you don't already I recommend a cover for the night - you've got almost 500 square feet of evaporative cooling area. I'd also check the support capabilities of your roofs... one gallon is 8.34 lbs and a cubic foot is 64.5.
As far as draining coiled tubing, disconnecting, and depending on your configuration, blowing it out with a good wetvac or an air compressor.
I used to work maintenance for an apartment complex and made a roof-top configuration out of sch 40 pvc painted black and laid it out and "zig-zagged" it horizontally so it would drain naturally, with the highest elbow being a "tee" with a threaded plug for air relief.
For a little comic relief: redneckpoolheater(dot)com
Good luck and I'd be interested in everyone's results!