Hi Zephans:
1. Why not use the grey abs plastic screw in clips for electrical conduit - I believe they come in 2" and look a lot nicer than those metal straps.

2. The 4x10 mats - which direction are the facing? By "above deck" I'm presuming you have a roof like structure above your deck you will place the mats on, so they lay relatively flat.

3. Are you able to build some sort of angled base for the 2x20's? Even a 20 degree slope from vertical would be very beneficial, because vertical mounting will really negate most of the gain you could see from those panels. Actually, when thinking about it, the 4@4x10 Which is twice what I have on my pool) is already giving you a good solar gain and should likely be enough for your size pool, I have 2 2x20's for a 30' diameter pool (20,000+ Gal.) and although the solar manufacturer recommends 3 x as many, I find the panels do a great job of extending my swimming season by over a month total.
I would try it with just the 4x10's and if you are not satisfied after a full season of use, add more panels at that time, should be easy enough since you are setting your plumbing up to accommodate easy changes.