First, a couple of web sites that may help:
www.powermat.com : has charts for each area of the country for solar gain.
www.flasolar.com : Sizing info and general information
http://www.fsec.ucf.edu/solar/testce...tr/tprpool.htm : Panel eff info

According to the site above, most panels are around 1000 BTU/sq-ft/day not per hour. So 80 sq-ft would rise the temp of your pool about 0.8 degrees per day above normal heating.

Generally you want about 75-100% of the pool surface area to make a substantial difference. The more the better. If you got close to 400 sq-ft, the temp rise would be 4 degrees. So over several days you can get a substantial rise but most installers do not guarentee more than 10 degrees above what the pool would be without solar. You can probably find you location on the powermat site for better temp rise info.

As for your other questions:

a) 20 psi is 46 feet of head which means the pump could raise the water 46 feet in elevation. The max head for your pump is over 90 feet so you should have no worries. After filling the pipes, the pressure drops because the water falling is negative head counter acting the rise.

b) For my system (automated), the solar turns on when the panels are 3 degrees warmer than the pool. This ok since it will continue to add heat but at a slower rate. See the powermat site for heat rise.